You mean is is again and again rebooting? Looks like a boot loop to me. What the hell you installed last time? Is your phone rooted?
Anyway turn off it completely by reinserting battery. Now you have to enter recovery mode, simply press and hold the Volume up button, the Home button and the Power button at the same time. Keep holding the buttons until you have reached the recovery mode.
In this mode you can navigate by "Vol up" and "Vol down" to move up and down and press power button to select. These three are navigation keys. Do "Wipe cache partition" and go to "advance->Wipe Dalvik cache". Your personal data/apps wont be affected, reboot your device. Please note that it will take some time to boot for the first as it have to rebuild cache required for system/user apps. It should work fine after that.