What was the last thing that you did before this problem occurred? It would help a lot to troubleshoot your issue if you give more information to your circumstances
Is the phone plugged in when you're trying to turn it on? Maybe the battery has gone bad?
Which iPhone and what iOS version are you broken on?
If you're using one of the older jailbreaks that are untethered, try booting up in substrate disabled mode. If the boot loop is caused by a tweak, then that should allow you to boot to home screen to uninstall the offending tweak(s) via cydia
Edit: oops, didn't realize someone bumped up an old thread