I had this problem with my 4th gear. All gears was cool except for 4th, it was like a slot to shift into 4th was not even there...I had to rebuild my tranny replacing the 3-4 sychro sleeve set. When I pulled the tranny and opened her up expecting a easier rebuild (cuz the shift forks within the tranny can cause similiar problems) I found that instead, all the teeth on my 3-4 synchro & its sleeves were badly damaged and rounded out.

That was my problem, it was the synchro/sleeve set. It can possibly be that for you or just the shift forks...either way, you're gonna have to open up the transmission if all else externally (i.e. shift linkage, shifter, etc.) is fine. It's definitely not the clutch (unless a spring came out of the clutch & got stuck somewhere), but most of the time if it's the clutch, you'll have problems with all gears not engaging like someone above already said. Good Luck!