Monitor Not Receiving Signal?

Hey, after reembling my old computer, I have come up with this issue; when I start up the computer, the monitor doesn't receive a signal. I have an old sceptre monitor, and it shows that it isn't getting anything from the computer. I tried taking out the AGP and putting it back in, taking out the RAM and placing it back in, but nothing worked. Also, when I start up my computer, it makes three long beeps lasting 1-2 seconds each with about .5-1 seconds in between. If you could help me out, I'd really appreciate it. Thanks.