it seems the gear box in the wifes gti has shit its self for no reason what so ever

put my foot on the clutch peddle and can get every gear with out any issue
when i lift my foot off the clutch peddle there is a grinding type noise coming from the box

it also has no drive

there was no bang or anything just revved up and makes said grinding noises

what am i looking at to get the box rebuilt but i know it depends whats wrong with it

oh and it has had a clutch so dont think its that due to the grinding