Sounds almost identical to what happened to my car last weekend. The car wouldn't go into gear with the engine turned on, but when the engine was off it went into gear just fine. I could drive the car, so long as I put it into the gear I wanted to drive in (1st or reverse) and THEN turned the car on. If the car was on, and I tried to put it in a forward gear I would press the clutch in and move the shifter forward, the car would creep forward. In reverse gear it would just grind.

The solution was that my clutch was totally worn out. No friction surface left. I had to replace my clutch then and there, since it's been fine. Sadly, if you're doing it at a shop, it's an incredibly expensive fix.

Edit: have you recently been noticing anything strange with the car? Clutch slipping? When I put throttle down, my clutch would slip as boost hit. RPM's and Boost would rise like mad, but nothing made it to the ground