PC no display

[Triventular]

So I have this PC I need to fix. The problem is that it turns on and the fans spin and keyboard and mouse lights are on but there is no display. I have tried plugging the monitor straight to the motherboard and its still the same. I have tried another monitor to see if that's the cause and its definitely not the monitor cuz it worked with another monitor I had. The weird thing is when i turn it on and see the fans spinning and no post, i press the power button and it immediately turns off as opposed to holding the power button a few seconds to shut it down. I noticed the cpu fan has a hard time spinning up and when it does spin it blows so much air. I tried cleaning it out as best i can with a brush and it still has a hard time spinning. I also noticed i need a new application of thermal paste cuz it was just not making as much contact as i think with the heatsink. I tried removing the RAM to see if it boots to and displays anything and its still nothing but another thing to note is that instead of just pressing the power button, i have to hold the power button for a few seconds. a total opposite of the thing that's happening earlier. Also the inside is so dusty and i can't clean it out exactly because we don't have a big empty space where I can use a blower to blow out all the dust.

I need to fix this as this is an office pc and I'm very limited to hardware here because there isn't exactly a stockpile of PCs here that i can harvest for parts. Thank you