Do you mean that after you restarted it doesn't boot up at all and nothing works?
or are you still able to access the PIX?
If it is completely not working, ie: not passing traffic, can't be accessed, then you would need to get an RMA of the PIX if you have SmartNet contract.
Pls also be advised that PIX515E is EOL and here is the EOL notifications:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/prod_eol_notice0900aecd8073fa36.html
Last day of hardware support for that appliance is July 27th, 2013.