Not able to engage gears when running

My bet is the clutch/throw out bearing. Jack it up and support it (safely) and get underneath for inspection. Make sure all the linkage is in place. (remember, it worked and then all of a sudden it didn't) Take the inspection cover off and check the throw out bearing. There should be a slight gap between it and the pressure plate fingers. Have someone push on the pedal and watch the action. There should now be a slight gap between the disc and the pressure plate surface.