Sony Ericsson Z750 hard reset
From the main screen in stand by mode Go to Menu by pressing right SOFT key, and select Settings press OK button, scroll down and select Master reset select Reset settings or Reset device. When You see warning message "Note: Deletes all settings and user data ..." select Cont. by pressing OK button. Phone will be restarted. Press OK to Cont.
Resetting phone. This will take a few minutes. Click OK, Memory cleared.
If nothing works: As last resort do a Sony Ericsson phone repaire with PC Suite. Use this method to hard reset AFTER INSTALLATION----> TOOLS > PHONE REPAIR