With the car off, can you use the shifter and move it in and out of any gear? If you can't, its your cables. I've never had an issue so I'm not sure what to look for, but if you remove the air box you can see the shifter cables on the top of the transmission. Id check to make sure they didn't come loose and that the bushings are good.
If you can move it in and out of gear with the car off but cant with it running, then its something with your clutch system.
See if the clutch is working properly. I had the seem's to be common third gear grind a bit all I had to do was bleed it and all was well. It should still pop out of gear even with weak hydraulics. SO-If the clutch is operating proper, like you can see the fork move 3/8's-7-1/16s or so at the slave. It's not that. By all means check the cables as dustin02rsx suggests. If a shift fork can break on PurEvil8975 it can surely happen to you? I'm mean he's evil so that's what he gets! Still the shift fork would be the last thing in line that's EZ to see. Best of luck dude.