Most likely the gear linkage. Something similar happened to me on an
Ibiza and it was the selector quadrant on top of the gearbox - the post that the cable attaches to had snapped off. This would prevent you moving the selector forks in the gearbox to get it out of gear. I got home by selecting second gear from under the bonnet, working the selector directly, then driving home slowly - it wasn't very far away.
As Mattyholmz says, open the bonnet and have a look at the top of the gearbox - where there are two chunky cables that should both be connected firmly to the selection mechanism on top of the gearbox. If either of them is waving about free, then that's your
problem.
Fortunately the replacement part is not too expensive and easy to fit - no need to get inside the gearbox, the retaining nut is on the top. This is a self-locking nut and should be replaced, not re-used.
The selection mechanism will probably need to be adjusted once you've replaced the broken bit.
If there is no obvious break in the selector mechanism, then it may be that a selector fork inside the gearbox has broken, which is usually fixed with a replacement gearbox, unless you are very handy with the spanners.