"Car won't start" - that tells us nothing.

Does the starter engage? (yes or no)
What sounds do you hear? (If the starter does not engage, what do you hear when you turn to start?)
If you turn the key to start, then let it go back to on - does the engine cooling fan slowly ramp up to high speed in about 30 seconds? (If it does, we know what is wrong - if not, we have more questions...)