I made a whole album with Sonar home studio and everything worked great. That project has been done for a while but now when I am ready to start the next one, I am getting no sound out of my headphones for overdubbing. In fact the music comes out of my stereo speakers just fine but not out of the phones at all. I'm sure it's something simple that I am just not seeing but if anyone can give me a clue to what I need to do, I'd appreciate it greatly.

Hi, and welcome to this forum. What kind of Audio Interface do you have? Does it have an option to select outputs in its software? Could you have routed your output (in SONAR) to phones instead of Main Outs? The more info you give, the easier it is to help.
All the best.

It's basically a YETI "Blue" mic that plugs right into the computer via USB. It was working fine up until yesterday when I tried to start a new project. The sound comes out of the speakers fine and when I use the headphones for other purposes they work fine as well. Just no sound coming through the headphones from the recording software. Same thing for old recordings. I can hear them fine out of the speakers but no HP....

I do not have this mic, so I am sorry I cannot help you. Hopefully someone else will chime in.
Best of luck.

Check your Preferences that the Yeti's headphone output is selected. Oh, Home Studio.
Go to Options>Audio and set the HP output as Playback Timing Master.
Good luck and welcome to the forum.