I have a Custom-Made PC from DinoPC.com. It's been working fine until today, I closed it down safley as I always do and took the case off to see how many PCI slots I have. I then unplugged the Hard Drive and plugged it back in. Now the PC cannot recognize the Hard Drive, However, It's definitely all plugged in as I've checked numerous amount of times.

On boot up I get "Loading Operating System" then it takes me to Windows 7 System Restore and Repair. However, none of the options work and ask me to restart by removing the Win7 disc and putting it back in on restart. This doesn't solve anything.

I need a solution to this as my PC is very important and I need it to be working again today. It's a 500GB Hard Drive. I did dislodge the RAM by accident, but I put that back in as I heard it click. Could this be the problem?

plug everything back in correctly remove power cord ,hold the power button in for like 1 min,then plug power back in ,try turning the tower on again ,
if nothing happens ,try removing the bios battery[silver battery about the size of a quarter ] from the motherboard for 5 or 10 min ,replace it ,this will return bios to defaults ,try booting computer again