Closed Sonar
 
Tried to reopen Sonar but nothing happens
 
Opened Task Manager to fine Sonar.exe still running
 
Tried to End Task but Access Denied!
 
Ending Sonar sometimes requires complete system shutdown and reboot.

Could be waiting on Bitbridge or interface drivers. If using 32bit plug-ins start by removing them.

In my travels I have found this problem to be due to a driver not being released (something held hostage).
I would like to recommend something's but I have no idea of your soundcard or other devices.
4000+ posts and your OP and sig is blank. you should know better my friend.
Best fill your sig to get productive responses.
 
As steve mentioned bitbridge can be the culprit but so can a device driver.
 
PS- you don't need to reboot every time. I figured out a faster way myself back in the day when this used to happen to me.
1- Simply log out of Windows.
2- Log back in.
3- Done.
 
This eliminates the whole reboot and cuts down minutes off the process.

This used to happen to me routinely when I was using an MAudio 1010. 
 
Once I switched to RME interfaces it never happened again (no association, BTW).

Thanks all. I'll try the user log off next time. This occurrence was most likely due to kontakt loading 32 bit instead of 64 (see other post).

Sonar holds onto the midi connections for as much as 10 seconds after the UI closes on my system. Just wait a bit and load again. You can always force-close the app in task manager, but that's not been necessary for me.

Another alternative is to turn your audio interface off after exiting Sonar. Of course, turn it back on afterwards.
 
Every once in a while the same thing happens to me with Sonar.

soens
....
Opened Task Manager to fine Sonar.exe still running
 
Tried to End Task but Access Denied!
 ...

 
A bit of a pain, but you should be able to end the process from the command prompt.  Steps given here:
 
https://www.techwalla.com/articles/how-to-kill-a-process-if-your-access-is-denied

The problem with ending the process is the next time you open Sonar (after killing it in Task manager)
Your devices are missing.
Logging off or rebooting is the best way to assure your devices will be present the next time you open Sonar.
I had this problem for quite a long time until I switched Soundcards and midi devices.

I have had this problem many times in the past. Try to end it but it doesn't actually exit. I have come back 10 or 15 minutes later to find it did end at some point.
 
I know it is likely a driver not releasing but knowing that does not help the frustration. Luckily I have not had Sonar crash in a while.