I turn the computer on, it works perfectly, the monitor has power but it doesn't turn on. I've checked with other computer the video card, the monitor, all the cables and they are fine. What else can it be?

I'm afraid it isn't working perfectly. In fact, it probably isn't working at all. If you don't get a single beep when you power up, the system is completely dead. That's why you aren't getting anything on the monitor.

Do you get a splash screen on start up, or any video at all, even if only briefly?

No, I dont get any beep or any splash. In fact, the monitor led has two colors, orange (when it has power but its turn off) and green (when its receiving signal from the pc) and it stays orange all the time, with no variation at all.
Can it be my Ram?, because i remove it and it happens the exact same thing...

I've had really goofy stuff like this happen to me before, it can be extremely frustrating. I had a P-II 400 Mhz that one day worked fine, the next day I wouldn't get any screen image at all. Just like you, I checked monitor/video card/etc. on other computers, all to see them work fine. For my problem it ended up being the IDE cables to my hard drives. I have a two-hard drive setup, and since the case is crap, one doesn't have a bay so it sits on the bottom of the case. Testing it a few times if I moved the HD just enough the CPU wouldn't boot, since that was the HD with my OS on it.

In my experiences, when monitor problems pop up where you aren't getting any output, it usually points you first to the monitor, but the problem usually lies in something that came loose or possibly a bad cable inside the system.