There are a lot of software issues with some applications that lock up the phone.
Try deleting newly downloaded applications and work backwards until your phone does not reboot again. Any paid for apps can simply be re-downloaded through the Android Market.
If this does not work, attempting a factory reset will be your next step.Factory Reset
1) Make sure you back up all of your contacts, pictures, etc. Factory Resetting could possibly delete all data off of your microSD card, and you wouldn't want to lose any of it.
2) Once done, from the home screen tap the Menu key
3) Tap the Settings
4) Under Settings, tap Privacy
5) Select Factory Data Reset
6) Tap Reset Phone
Once done, your phone should be restored to factory defaults.
If for some reason your phone has decided to continue crashing and rebooting, you must perform a Hard Reset. This will undoubtedly remove contact information.Hard Reset
- Remove the battery and replace it.
- Press and hold the Volume Button
- Press and hold the Power Button until the phone powers on.
- You should see a simple menu showing options such as Fastbook, Recovery, Clear Storage and Simlock.
- Using the Volume Down button to navigate, Highlight Clear Storage
- Press the Power Button to select Clear Storage
- Confirm your decision with Volume Up for Yes. If you feel like backing out, press Volume Down.
Remember, everything will be lost from this point on; so be sure you have EVERYTHING backed up that you do not wish to disappear. This will return your phone to it's original, out-of-the-box condition, software-wise.
If even, after the Hard Reset your phone still reboots, contact your Customer Service department for a warranty replacement.