Erlend, you need three hands for this, so unless you're exceptionally gifted, get someone to help!
Put the neck over your knee, fretboard down and press down on the body and the headstock to take the stress off the truss rod. Obviously don't use too much force, you don't want to break anything! While you're doing this, try loosening the nut. If it doesn't turn, sounds like the nut is seized on the rod, probably through cross threading or having run off the threads. If it does turn but doesn't loosen, sounds like the rod is broken or loose at its anchor and is binding in the channel. I would guess something mechanical has gone wrong with the rod, probably the nut overtightened on the rod beyond the threads. Without knowing more about the type of rod fitted it's difficult to advise further. I would agree with Twilight about a half turn being a fairly huge adjustment, though it depends on the guitar, type of rod and sensitivity of the neck, and realistically up to a quarter turn should be enough for most adjustments unless the nut is actually loose. If there is genuinely a problem with the rod, it's a fretboard off job I'm afraid.
