bear with me, but if you have a low post count, and made a car audio question thread, you get this post:
this forum is primarily for installing computers in cars, so audio advice is on the slim side.
if you are here strictly for a car audio advice, there are plenty of dedicated car audio forums listed in the sticky at the top of the car audio section:
onto your question:
the question is too vague, and i imagine that most car audio sites won't be too willing to help either.
post as much info as you can, otherwise, its like asking us why your car won't start when you turn the key-- there are a bunch of things that could be wrong...it could turn out that [deep breath] the-cat-chewed-the-wires-while-the-dog-chased-it-and-in-the-process-broke-the-fuel-lines-and-oil-plug-so-not-it-won't-start...
check your power wires, and speaker connections, and if your still not sure, take it to a car audio shop for some ideas