Happened to missus's cousins Jetta. (same car)
You'll have to lift the bonnet to the max height it will go, then slide your finger toward the catch and you should be able to move the catch just enough to let the bonnet pop open, you might need to lower it slightly just to take the pinch off the lock catch.
Once it's open, as Alan said, get some WD on it, they get caked in crap from the road, so a quick spray and clean will set it right.