2008 Civic SI
I pushed in the clutch to start the car and heard a pop like something broke, tried putting it in gear and couldn't. I still have pressure on the pedal so I'm guessing the CMC is ok since the pedal comes back up, but could it just be the slave cylinder? Thanks

 

Have you tried starting the car with it in gear and clutch in? Does it jump?
How many miles on the clutch/system?

 

I have not tried starting it in gear and it has 120000 miles.

 

You probably have a broken pressure plate spring from what you are describing.