Just by accident I discovered way to recover from some situations when audio engine stopped (with drop) but refused to start again...
No matter how many tries I did. Pressed start/stop engine play project but it was not running, no error, nothing, like it would be empty... Removed all vst, tried again, nothing...
Normally in such case I would close project and start again what is super annoying. But just tried to run some undo/redo and magically it cured engine and even restored my vst... :)
 
Probably not to be used in every case when engine don't want start but maybe worth to try before reloading everything.

Interesting!

Usually the audio engine stops because something is going on in the background and has interrupted the engine. It can't access buffers etc. If you wait long enough what ever was doing this will finish and sort things out. 
So you keep clicking you actually only make things worse as your stacking more tasks. 
What you really need to do is run Latency Monitor and find out why your loosing the audio engine. 
On a properly set up PC you should never have this issue unless you exceed your systems capacity, then you just upgrade components to fix that. 

Johnny, I am afraid is not such simple.
 
1. If it were as you described why just closing and reopening project would solve issue?
2. Waiting longer time should also resolve issue (and let start ) but it is not case.
3. When trying to start play nothing is happening... No error message but also no any action...
 
BTW, I was not just clicking 10times per second in buttons ;)

OK that's different.. like I said "usually" 
So next place to look is your Audio interface and drivers. What do you have? and which driver mode. 
Also a common issue is HDMI audio drivers taking over. Most people find disabling that in device manager a good move. 
 

I am using on this machine RME FF400 and its ASIO drivers. Audio from monitor disabled at device manager.

One method that works sometimes for me: power off your soundcard for a while(!).
SONAR will notify that it has found a new audio driver and asks if it may stop the playback. Yes => audio engine is alive again.
 
This is not any generic fix method but may help sometimes.
My interface: RME UFX / ASIO, USB mode.
 

Thanks, Panu, sounds promiscuous for such cases.

Piotr
Thanks, Panu, sounds promiscuous for such cases.

Hey! did you mean "having or characterized by many transient sexual relationships?" 

MFanning

Piotr
Thanks, Panu, sounds promiscuous for such cases.

Hey! did you mean "having or characterized by many transient sexual relationships?" 

Well, looking deeper at disconnecting and connecting audio interface one could have different associations :)
With all my poor English I only hope nobody feels insulted by me :)