On a starter than is "stuck"... meaning it's going out and getting weak (brushes or something), a couple of blows with something heavy will usually get it spinning again as a temporary solution until a replacement can be fitted. I have been utilizing this for a month or so on one of my e30's that has a bad starter. You'll need someone to turn the key for you while you try to knock the starter back into life. I use a 3' handle from a floor jack...

The above posts are an excellent way to diagnose as well. The method above is just very quick and easy to try so may want to try it first. No need to go all Schrazenegger on it, just give it a couple of good taps... don't want to bend the starter housing.

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