P990 Soft reset Reboot
Press and hold the Power button for about 10 seconds
Master reset Sony Ericsson P990i
Use Master reset to reset your phone
Control panel -> Other -> Mater reset
Important: All user data, as well as pre-installed media
files and application data files (including, for example,
backgrounds, ringtones and pictures), will be deleted
and settings will be set to the state they were after you
selected the language. The selected local language will
be restored.
If you want to keep installed applications, unmark the
Delete user installed applications check box.
Format internal disk Sony Ericsson P990i Symbian hard reset
Switch to P990 service menu
Hold down Space key on QWERTY hard keyboard
Jog Dial Up / Press * + Jog Dial Down / Jog Dial Down / Press * + Jog Dial Down / Press *, then choose format internal disk
OR
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
http://www.sonyericsson.com/cws/support/softwaredownloads/detailed/pcsuite/aino?cc=gb&lc=en