Mersiegai

Hi, I have a spare computer that my friend gave me that I keep in the spare bedroom, and it's not used very often. Before she gave it to me they wiped the hard drive and installed windows 7 on it for me, and it was a little slow but worked fine. I had not used it for a while and turned it on today to use it and the power goes through and the fan works but the light doesn't change from orange to green, just stays orange and I get 3 long beeps, and nothing. I removed it and hooked it up to my other monitor just in case it was that or a connection, but it still wouldn't work. Does anyone have any clues as to what could be wrong, or why t has suddenly stopped working. Thanks Marilyn