Car won't start since I put in a stereo

Double check the ground wire where you connected, or should have connected the radio. Just a wild guess, but I have done that before.

Check the fuses under the hood. You may have crossed a wire and blown one. If you have an after market anti theft system, it may have cut the fuel. Or just by chance, your fuel pump has failed. I take it that it cranks over but just won't fire up? Like not getting fuel? Remove gas cap, put your ear to the hole, have someone turn key to Accessory position. Can you hear the fuel pump hum?