We've had a Gateway 816GM
Bill,
If the hard drive is still good, your files are safe. Replacing the motherboard may mean you will need to reinstall the operating system so you will have to get the files off before you do that.
Now, even though it could be the motherboard, it could be any number of things. If I read your post correctly, nothing happens at all now? You push the power button and nothing? This to me sounds like the power supply. These go out quite frequently and a failing power supply can cause it to lock up. When the power supply fails completely, then you don't get anything. These are relatively inexpensive.
If a new power supply does not solve your issue, then start by removing all internal cards, floppy drives, cd drives, hard drives from the computer. Actually, you can do this first to make sure it still doesn't do anything before replacing the power supply. Then try and turn it on only with memory, cpu, cpu fan, and the onboard video (or video card if it doesn't have an onboard video port). If it does work, then you can add each item back one by one until you find the culprit. However, in my experience as a PC tech, this is almost always the power supply. Probably something like 95 % power supply, 4 % motherboard, and 1 % other. Good luck and if you need any further assistance, let me know. Also, I would appreciate it if you could give this solution a rating of fixya. Thanks! :c)