Ok, Thanks-but that does not work it says device does not support current audio format...so I am a wee bit stuck. The thing is that it worked fine for the 2 years or so before an..d I have not done anything diffferent.
 

If you recently updated Sonar, it may have reset your preferences. 
 
Please let us know what audio interface you have.  One easy way:
- Right click on the volume control icon in the Windows system tray
- Click "Playback Devices"
- Tell us what it says (the second line should say the make and model of the audio interface)

Reinstall the driver for your device, windows sometimes removes them… 

You still are not telling us what audio interface your using... that makes answering this issue guesswork. The more info we have on your system the faster we can help you resolve your issue. Most of us put this in our signature to save time. 
See Jonesey's , lrared , dlion16 and my signatures as examples. 
Doesn't need to be detailed, just the core of your system. 

Ok I see what you mean-sorry! Just new to all this.
 
Sonar Artist 21.6.00 BUILD 25 Glouscester x 64
Playback device : Realtech High Def Audio Defualt Device
Recording : Realtech High Def Audio  Stereo Mix
I also have NVIDIA HDMI Output (Port 0) and I also have NVIDIA HDMI Output (Port 1) which are disabled
 

gillrussell
Ok I see what you mean-sorry! Just new to all this.
 
Sonar Artist 21.6.00 BUILD 25 Glouscester x 64
Playback device : Realtech High Def Audio Defualt Device
Recording : Realtech High Def Audio  Stereo Mix
I also have NVIDIA HDMI Output (Port 0) and I also have NVIDIA HDMI Output (Port 1) which are disabled
 

There's your problem.
 
Without wishing to sound unkind, this isn't a proper interface and will give you no end of grief trying to get it to perform well with SONAR.
You don't need to spend a fortune on a decent interface - give us some idea of your budget AND your intended use and I'm sure some recommendations will be forthcoming

Thanks -I did wonder. I  am and installtion artists, not a musician .... and  use it for mixing, editing and recording sound, and I also use effects . I had a better sound card (?) before when I had a Sonar Producer version and was using it to make surround soundtracks for my work  (light pieces /surround sound -examples at www.gillrussell.co.uk ) . But when I upgraded my computer from XP it did not work so I bought Sonar Artist -budget not great ! Really appreciate some advice on what interface to get, and how to install it etc. I probably don't need a very sophistacted system. Thanks so much for your help on this. I wish I had asked before I bought !
My computer is
Windows 10 home
Version 1607
OS Build 14393.576
intel (R) Core (TM) i7-4790 CPU@ 3.6GHZ 3.60GHz
Intalled Ram 16GB
64 bit operating system, x64 -based processer

Glouchester was released July 2015. If you have an active membership consider updating to at least 2016.11. There were significant improvements to the WASAPI mode with that release specifically for configurations like yours.
 
Your membership status is shown in the SONAR about screen and https://www.cakewalk.com/My-Account/Membership
 

Gill, are you generating all your sounds within Sonar or are you using microphones/line inputs? Onboard sound cards are very minimalist and not much good for production of any kind. You would be better off with an external USB sound card. There are loads out there at varying prices. If you are not using microphones, you don't need anything too high end and expensive with great mic preamps or anything. I use Focusrite gear almost exclusively, but a lot of their stuff is way more than you probably need. This is probably the most budget one they do and may be all you need. While you can get very cheap and cheerful ones, you will often have difficulties with them such as bad drivers, latency, and substandard audio quality. So get something from an audio brand you can trust or scour the web for reviews.

Ahhh ...I guess I did not know... what I did not know !
I am not recording straight into the computer -for recording external audio I use a Marantz (with good mic) then import the wav file  to Sonar. I have also imported  sounds from my keyboard  directly through line in but quality was bad. Sometimes I record things  from live internet within computer. I am also updating Sonar as I write .  I can probably afford the budget external sound card you recommended as well. I am not used to going on forums etc. so please tell me how I can thanks you all for the help !