start with the simple things first mate
power down, unscrew thcpu and remove the cable from the back of the drive
replace them and then power back up
ensuring the are seated properly
if that dosnt work
go to my computer right click and choose properties, then hardware, device manager and look in there for any yellow explamation marks
if there some thing in there, post it and then wait for an expert in here to tell you what it is!
hope that helps