Also depends on what happens when you try to start it.
When you turn the key, does it make a loud, fast whirring/spinning noise but the engine doesn't sound likes it turning over? This might be due to a chipped tooth on the flywheel or starter gear. See if the engine can be turned by hand using the crank pulley or use one of those big wrenches that fit in there to just turn the engine just a tiny bit. Then try again.
The tip on hitting it with a hammer is another easy attempt and has worked plenty of times in my experience. Things get stuck and jammed up all the time in old starters.
Also check that the starter motor is bolted snuggly up to the engine block. If it's a little loose, the starter gear may not be making contact with the flywheel. This happened once to me.
If when you start it, it does nothing more than make click sound (if that), it might be an electrical problem. Check fuses to make sure. These are all little things that might have happened which you can fix easily with having to pay a garage.
Don't try to push-start an auto, it just won't work.