If I'm understanding you correctly, I had similar happen to me old 93 4.0 5-speed while on a business roadtrip. I had downshifted into 4th for a large hill coming. Upon cresting the hill I threw the clutch and popped her into 5th. Seconds later the shifter shot out. Thinking a possible amateur mistake shift, I reattempted 5th gear, same thing happened. I nursed her to a dealership where my 5th gear synchro was shot. Had synchros and tranny R&R'd.
If you heard a whining, whirling, drone coming from the shifter while in gear, that's most likely your throwout bearing going/gone.
April 7, 2005 the arrival of:
'04 FX4 Level II - 5spd manual...me likey!