Okay here's the thing. I recently returned to the states and wanted to use my old PC Gateway 838GM
Now I didn't have a monitor so I borrowed one from a friend a Philips 170b2b
Plugged it in and turned the PC on. Fans turn on and LED are blicking. And all I get is No Video Input on the screen. So what I tried is; I took out all the memory sticks(except one), unplugged the HD, CD drive and the video card. Hooked up the monitor to the intergrated video card and the it turned on And everything turned on and, Windows booted up.
Then I turned it off again to see what can I put back in, but when I tried to turn it on again it gave me the same **** with No Video Input. So what should I do now? Maybe I plugged something wrong or I unplugged something I shouldn't have? What should I do?

Put everything back in, except the graphic card; if it boots and works in stable manner, you have your suspect i.e. graphic card. Otherwise, you should take step by step approach, and add one thing at a time until you find the hardware which actually is causing this issue.

It worked with one memory stick and with the HD pluged in. But after I turned it off and then turned it on again with the settings it didn't turn on.

Please ensure that the system is dust free.

Clean the contact edges of each RAM module with eraser, and retry.

Also ensure that all connectors are properly seated.

Did that and replaced the PSU. Still nothing.

I'm assuming here that onbaord vga isn't working/enabling computer to boot either?

Try resetting BIOS; you should be able to find information about resetting BIOS in motherboard manual. Also ensure that your motherboard is not shorting (look around to see nothing is directly touching casing etc.)

If that doesn't work, you probably have motherboard issue.

Tried the onboard one and already tried resetting the BIOS. And what do you mean by shorting?

I mean motherboard may be touching the metalic surface of casing. Another idea can be to remove motherboard from casing, and then try booting it again.

It seems that there are some parts where the case is touching the mobo. But I'm afraid to take it out, is there a right to do it or is just take out screws and pull it out?

Take out the parts, like graphic card, then screws. Depending upon the space you may have in this casing you may be able to pull out the motherboard without having to disconnect all connectors, however, do it with caution. In case, you do not feel comfortable about it, I'd suggest to take this computer to nearby tech shop and ask them to diagnose the issue for you.

Ok it was the mobo touching the casing. Solved all the problems, thanks!

Glad to hear that, and happy computing