That's the equivalent of saying "There's a problem with my car what part do I replace", need waaaay more info.
Answer a few of these:
Are all of your drives listed correctly during POST?
If not are they listed correctly in the BIOS?
Are all of your drives starting up (can you hear them spinning)?
What's the EXACT error message?
Are there any other messages?
What kind of system do you have?
What kind of parts are in the system?
Did you have any issues before?