Settings > System Settings > Format Utility > Format Hard Disk Doing this will delete all your data, including all game saves. Back up what you want to save before proceeding.
Or by going to recovery mode. One time my PS3 Slim's 250GB HDD wasn't working, I phoned PlayStation but they were useless as hell, so as it didn't format on PS3 itself I connected to a laptop and there it formatted, then connected back into the PS3 (format again and all working fine since then). It was my fault primarily, I turned it off incorrectly.