After working on a project for ten hours today, my block of flats received a power cut that lasted for a couple of seconds. My PC (and everything else) came back on. Went past the BIOS and back into Windows. Upon loading Cakewalk, my project was in the same state as it was ten hours ago. I regularly saved  my project, and have auto save enabled too.
 
So where has all my work gone? The momentary power cut wouldn't have wiped my drives, as my project is still there; but without today's work on it.
 
Does anyone have any ideas as to what might have happened?
 
[Addition] Thanks to the input from the replies on this thread, I have narrowed it down and identified my error. TL;DR: I set up a Project Template and then started recording audio; as I would in a project. As it's not my habit to make templates, I assumed (wrongly) that I was auto saving a project, because that's usually what I do (habit). Thanks to the power cut, I learned this early. The suggestion further down this thread is a good one: Prompt: 'This is a Project Template(.cwp), all Audio will be lost, do you wish to continue, or save as a Project (.cwp) file?' Cheers to you all.

Well you said your were saving as you worked. So the project should therefore be the last save.
 
Only other possibility is you have opened the project in one location and all your saves were placed elsewhere. This can only happen if you use the "save as" function. 
 
Use Windows Search for the CWP name and look at the time stamps. 
 
And to prevent this from happening again always perform a "save as" to a back up location after spending any amount of hard work into any file. I used to use a second DATA drive for this reason. I now have been using One Drive if the songs are at the midi only stage. Once I start recording Audio there will always be 2 locations for each song file. And then those are backed up every once in a while to a 3rd location. 

Cactus Music. That's the thing, I've done exactly as you described, including the 'save as', I do it religiously. I have Sonar (or Cakewalk by BandLab, now) pointed at my chosen directory for project files in the Folder Locations preferences, to a separate drive. When I go to that location, the only recent save is 5 hours ago.
 
I don't understand.

how's your hard disk configured? (can't remember the exact windows terminology) but it can be set for performance, where data is left in the disk buffer and flushed from time to time, or configured the other way where data is immediately flushed to the disk... see here https://www.tenforums.com/tutorials/21904-enable-disable-disk-write-caching-windows-10-a.html
and if you haven't overwritten the project file yet, check the timestamp for last modified
/hth

Found it! Now it's saved as a Project Template, with some of the audio missing. I keep getting the old 'Silent Buses' dialogue as well. Trying to put 'WarnSilentBuses=0' Into the initialisation file no longer works; press 'apply' and the text just typed disappears . I hope this is transition teething problems for Bandlab. 
 
So today, my templates have been saved as projects, and vice versa. Now Cakewalk reports missing audio and silent buses when I open that project, so I imagine that audio data has gone elsewhere too. I think I will spend another ten hours trying to find that data. 
 
The only audio data I lost was an auxiliary send track from the metronome. I spent the day getting project ready,; I can do that track again. The thing that concerns me is that when I record some music, where will my data go?
 
I was going to have a go at some 70's/90's Metal, as I'm enjoying the challenge from years of Punk and Ska rhythm guitar to fast soloing, but now I think I'll do a cover of 'Bewitched, Bothered and Bewildered'

Just to check the obvious, if you search your machine for *.cwp files, the most current (anywhere) is 5 hours old? I am not sure if autosaves go to your project folder (or how they are named), but not having anything anywhere is very odd.

mettelus
Just to check the obvious, if you search your machine for *.cwp files, the most current (anywhere) is 5 hours old? I am not sure if autosaves go to your project folder (or how they are named), but not having anything anywhere is very odd.

Cheers. I didn't search for *.cwp files (good tip for next time). I was typing my last post whilst you posted this one, my post above semi explains it???

pwalpwal
how's your hard disk configured? (can't remember the exact windows terminology) but it can be set for performance, where data is left in the disk buffer and flushed from time to time, or configured the other way where data is immediately flushed to the disk... see here https://www.tenforums.com/tutorials/21904-enable-disable-disk-write-caching-windows-10-a.html
and if you haven't overwritten the project file yet, check the timestamp for last modified
/hth

I have my drives set so they never power down, no matter what other setting is made. I have my PC set up for performance, and have streamlined it as much as I have the knowledge to do. I'll read the article you linked, thank you for that. It seems that there is a conflict/confusion with file locations and/or extension names. It's either that, or I made a wrong turn, and saved a project as a project template. 
 
Thank you.

Geoff I'm glad you didn't loose everything.
CbB has an auto save feature. In the beginning there was some resistance to using it because some users were concerned it might auto save and do a similar thing to what happens when you load virus software. Automatically jump in to auto save and stall a project. Thus far this hasn't happened to me. I'm saying this for the benefit of anyone else who is either on a large project or beginning one. 
I've been there, done that and wasn't as successful. I had to learn it the hard way.

Starise
Geoff I'm glad you didn't loose everything.
CbB has an auto save feature. In the beginning there was some resistance to using it because some users were concerned it might auto save and do a similar thing to what happens when you load virus software. Automatically jump in to auto save and stall a project. Thus far this hasn't happened to me. I'm saying this for the benefit of anyone else who is either on a large project or beginning one. 
I've been there, done that and wasn't as successful. I had to learn it the hard way.

I use the auto save feature and press CTRL S regularly. I'm still learning. Probably overlooked something. Trying to work out what, though.