Computer wont run and beeping sound comes up.

Hi everyone, my Gateway computer ive had for 5-6 years wont run on start up. This is what happens. I turn the computer on and after 3-4 sec it restarts as in "makes the noise of it turning on again" so basically it makes the noise twice when it should be one. After that 2sec later is starts make a 3 sec beeping noise every 7sec. I think the first beep is longer but after that they last for 3sec. My monitor is completely black. The on light is on so its not like it cant turn on fully. Anyway what may have caused this. And also yesterday when i used it for 30min browsing the web it froze on me completely and i turned it off by holdig the power button. Last time i ever used it. Its spec's are 2GB DDR2 RAM, GT9800, Amd athlon x64 4200 2.2GHZ, the rest idk. Thanks to all who try and help.