Latitude E6400 not working

Hi, i'm the new guy here.

So my uncle got this old latitude e6400 way back at 2007 when he was working at tech store. It was turned off since 2009. My uncle found it somewhere at the attic and have it to me. So i turned it on, i got welcomed with "enter password". I turned it off and cleaned it from the outside since it was very dirty. Turned it on again and... nothing, power button was lighten up, heard 4 beeps from dvd drive and after some seconds it turns off. I turned it on again, for no reason clicked it again and fan started spinning, also after some seconds it turns off. I also tried it on battery. Something changed, later it showed the dell codes, caps lock and scroll lock were lighten up and numlock was blinking (i couldn't catch it on camera). Because of 4 beeps, i changed ram and nothing. This numlock thing made me think of cpu, bought new one aand nothing changed. I think it's the motherboard. Any suggestions? Also sending youtube link of how it looks like https://youtu.be/yHhGSTIlFI4

thanks