If the computer has POSTed before, then something has failed. It sounds like the RAM is bad. If it has more than one stick of RAM, remove all but one stick and then see if it will post. If it does not, remove that stick and install another stick. If it will not POST with any stick, then either:
-- all your RAM is bad or
-- the motherboard is bad.
If the computer has never POSTed, then the cause could range from a defective motherboard, to defective RAM, to a defective Power Supply.