HDDs should be handled with care. I'm assuming you didn't drop your computer or hit it hard somewhere but you should try opening the back cover to re plug the HDD. Sometimes cables can cause this problem.
P55W-C5204 has one big cover at the back and letting you reach all the components with an ease.
If you're worried about the warranty or have no idea what's going inside do it on your own risk.
Your hdd is placed near your battery. Once you open the back cover remove your battery first. After 30 seconds unplug your HDD and plug all back in (always unplug the battery first and plug it the last)
if this didn't solve your problem try reaching bios to confirm that your computer is reaching the drive you have to boot from. Even if it's not showing try changing the boot device on BIOS settings.
If you can reach the drive and yet still can't load your operating system Look for "UEFI windows from USB" on google