Is it just one beep, or several? If its several, it should be a series of short and long beeps. Example: three long beeps, or one long, two short, etc. If it's just one beep and nothing, you need to open your computer and make sure that everything is seated right on the motherboard. Especially the CMOS chip, if it's socketed.

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One beep error's are unique to AMI motherboards, and indicate RAM memory problems. Try this: remove the memory sticks from their sockets, inpect the edge connecters on the RAM to make sure it's not dirty, tarnished, etc. If it is, use SMALL amount of rubbing alcohol and a stiff bristle brush to clean the contacts. Let the sticks dry for a while. In the meantime, use compressed air to blow any dust, or whatever from the sockets the RAM sticks sit in. Reassemble and try to boot. If it still does not work, you probably need new memory.