Not recording audio? Flat line.

Hey, im new to the forum, and im a newb. Ive been working on a project for a few days, everything has been going smooth until just a few hours ago i tried to track a guitar. When doing this the audio is flat lined. I check my vst connection, device setup etc. So out of curiosity i started a new project and set things up as I normally do, created an audio track, and it worked fine. Any suggestions of what I should try? Thanks.