suddenly i can't get any sound into my sonar 8.0 from either my yamaha keyboard or my m-audio midi keyboard. i've tried everything, restarting devices and computer, changing ports, checking the sonar settings etc. nothing helps. i wonder if i should try reinstalling the sonar, upgrade to a newer sonar, or replace my 7-year old pc laptop. any suggestions?

Dumb question - did you input echo?
 Also, are you turning on your device before starting Sonar and checking the box in preferences to set it to input?

yep, did all that. same procedure as allways, the only difference is no sound.

I would try reinstalling your midi device drivers before going the entire Sonar route.
 
Is this a new project or an existing project?  Does any audio sound?

My first test is to play the synth from the GUI if possible,
Not all soft synths have this feature but the drums and TTS-1 allow preview with the mouse. 
If the GUI doesn't play then you work from there. 
If it does sound then your midi input is not set correctly. 
 

Paul, specs in your sig are a good place to start  If your running win 8.1 there are known issues with certain devices  and the way 8.1 implements the general midi driver this is a departure from win 8 which does not have this problem
 
If you have not made any hardware/software changes recently I would download something like midi-ox or another software midi patchbay/editor  which will give you a graphical representation/readout of what's going on in midi land and may save you the headaches of re installing things without a logical ordered process in place to troubleshoot from 
 
Cheers, 

I don't have Yamaha MIDI controller keyboard, but I'm experiencing same problem with AXIOM Air 25.  For the life of me, before latest Windows 10 and Sonar Platinum update, my AXIOM Air 25 was able to get MIDI input to work correctly.  Now, no matter what I've tried, I cannot make any sound from the MIDI controller (which is AXIOM Air 25).  Of course, various VST programs' virtual keyboard and whatnot make MIDI fine.  My problem is that the MIDI keyboard controller isn't talking to Sonar alright.  The funny thing is that whenever I turn off and turn back on the MIDI controller keyboard (which is AXIOM Air 25), Sonar complains that it wants to reconnect the device again.  This means AXIOM Air 25 is being recognized by Sonar, but why no MIDI input playback at all?  I've tried usual steps such as reconfigure MIDI, restarting computer, and whatnot, but nothing is solving this problem.  I'm downloading Cubase as we speak just to see if the problem would exist in another DAW with my MIDI controller.  Sigh.
 
Additional information:  I'm using latest Sonar Platinum (2016).  Installing this from Command Center.  AXIOM Air 25 MIDI keyboard controller doesn't need driver update.  Windows 10 rig.
 
Update:  I finally fixed the problem.  The Tempo button near the LCD screen for Axiom Air 25 got turned off somehow, and all I had to do was turning it back on.  I don't usually play with various features on my MIDI controller, because I only rely on the keys to interact with the VST programs.  Nonetheless, I think the Axiom Air 25 MIDI controller got reset somehow, and the Tempo button got turned off.  I have never had to deal with this problem before, and so I thought it was software related.  It was hardware related.  After figuring this out, now I can make music with a passion again.