I guess you mean the light close to the power button, if not let me know, that's the hard drive indicator, anyway, according to gateway documentation there should be either an error message on screen or a beep alarm, it depends on the kind of beep the kind of error.
But if the error message is in the back of the server, next to the power cord plug, then the problem is the power source, I recommend you to open the server and try to identify physically what is the problem.
Another option, and that is only if you are very lucky, sometimes you need to unplug your server from the power source a few minutes and plug it back on again, it sounds crazy but sometimes if there were a power surge you need wait for some electronic devices to discharge, again that is very rare and most of the times you need to replace the power source.
And my advice is, if the server is still under warranty, call gateway, I guess it is not because you are here :D, hehe, anyway I had to say it.