Today it happened to me I couldn't put it in 5th. Other gears worked fine (although seemed not so smooth). Tried several times, then pulled over, lifted the bonnet and manually moved the selector (or what's it called) around a bit. After this the problem was gone completely. Any idea what it might have been? Some dirt stuck somewhere maybe? Or is the gearbox going? :'(

Sounds like a problem with the linkage, not the box.

Had a gear selection problem on a 1.9 TDi Octavia some time ago. The fault turned out to be a broken plastic part on the linkage on top of the gear box.