1) The first thing to try is to power off the phone, remove the battery for at least a minute, reinstall the battery, power on.
2) The next step would be: from within Safe Mode, remove the applications/updates most recently installed and restart again.
3) Perform a factory reset
a) Soft Factory reset: Press Menu -> Settings -> Select "Backup and Reset -> "Factory Data Reset" -> "Reset device" -> "Delete all" : Your phone will begin the process of resetting.
b) Hard Factory reset:
1. Turn the phone off
2. Press and hold the Power, Home and Volume Up buttons
3. When the phone vibrates, release *only* the Power button
4. When the Adroid System Recovery screen is displayed, release the other buttons
5. Press and hold the Volume Down button until you receive the message "Delete All User Data"
6. Press the Power button to select that option. The phone will begin resetting
7. When you receive the message to reboot the phone, press the Power button and the phone will reboot
If at this point you still boot into Safe Mode then you appear to have a hardware problem which will need service or replacement.