First of all you could try to remove the SD card from the phone if you have one inserted. Restart the phone and try to open the camera. Also make sure that no other application running in the background is using the camera application. You can try to start the phone in safe mode:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
If the camera still says it's unavailable I would suggest that you have your phone examined and fixed at a service center. Please contact your local Xperia support team for more information about your nearest service center:
http://www.sonymobile.com/global-en/support/contact-us/
