Shifter interlock?

this is by no means a cure, but for the time being you can get it into gear with a bit of patience ....and timing. as your turning your engine over..at the point it turns over put it into gear-it might take a few attempts but you should get a gear. i have had a couple of beemers that i have dismantled do this(both e36 compacts) . remember keep your foot on the brake. once its started it may remedy itself. timing is crucial with this trick. :-) just before your engine fires it will slide into gear.

my trans am had that problem, it was a transmission mount. Not sure about a bmw, it could be a lot of number of things.