If your phone doesn't start, remove sim card and memory card if have it.
Then, place the battery without sim nor memory card and do a master reset.
This will format your device, so, all pictures, music, contacts or phone numbers, messages, games, notes and all things stored in the mobile phone will be erased, so proceed at your own risk.
First, make sure your battery is fully charged. If the battery is drained out, it may take up to 30 minutes of charging before the battery-charging icon appears on the screen. Remove the battery during 30 seconds and place it again.
Press the On/off key for a minimum of 10 seconds to turn the phone on.
It should work.
If this does not work...
Try to update software phone by firmware upgrade using Sony Ericsson Software Update application or PC Companion. Connect the data cable and follow the steps in the wizard.
Sometimes, to be recognized by the Software updater program, you should to keep pressed (C) key or (<-) (Back key) or perhaps both keys and connect the data cable.
If anyway it does not work...
Try to turn on your phone by keeping pressed (C) key while turn on your phone. If nothing happen, keep pressed (C) and back key (<-) at the same time while turn on the phone. Some others require only the back key (<-) pressed while turning on the phone.
It should format your phone.
Good Luck.