I think we need some more symptoms. Do you mean that the starter turns the engine over, but it doesn't fire, or that the starter makes a noise but the engine doesn't turn?
If the former, it's the usual suspects -- no fuel, no spark, too-slow cranking, timing out, etc etc.
If the latter, allan.g0ias's suggestion could be it. Other possibilities are a very weak battery, so the mechanism doesn't engage (this gives a rapid rattle), or a lack of lubrication on a sliding cog, or stripped teeth on starter or flywheel. Or more things I can't think of at the moment. A loose starter motor, thus incorrectly aligned?
Is it an automatic? If so a bump-start may not be much use.
Good luck.