Listen to the HDD. Is it grinding away like it's booting?

If it is, then the tube in your old monitor might be blown. If you dug it out of the basement, then it could simply be that something fell on the monitor during its long sleep under the junk. The glass tube inside is rather delicate, and can be damaged by falling objects and sudden impacts. That light changing to green on the monitor indicates that it is indeed getting a video signal from the Macintosh. Sounds like you may have a blown tube. Sorry man :-/

Good news is that the computer itself might be perfectly fine