i put in a stereo and now my car will not start up what could it be?
You may have cut the wrong wire or popped a fuse. Check the fuses first and if they are good, pull the radio and check your wiring. Make sure you didn't pinch wires during the install.
Agree with 10speed's answer. Did you disconnect the battery when doing the radio install? If so, you may want to check that as well.
Unplug the battery negataive cable and attach it again. If you didn't take it off during the install, you might've shorted something and just need to reset something. Otherwise fuses and check all connections to make sure they're not shorting.