When you press the clutch this causes the transmission to separate from being driven so that you can shift the vehicle. If you can't shift it could be a problem with the transmission itself, the linkage of the shifter or may simply need a clutch pedal adjustment.
I would go with the pedal adjustment first, since it is easiest, and then go from there. Good luck.
〉 Answered on Apr 3rd, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines