Question PC powers up but displays nothing

Nov 14, 2020
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First of all, sorry for posting this i believe this has been posted so many times, but my problem is slightly different so, thanks anyone in advance for reading.

So i was considering buying a new PSU because my GPU is on arrival, so i openned the case to see what kind of PSU would suit me the best and when i closed it, connected all cables again, powered it up, my display shows nothing.
I unfortunately dont have that little buzzer so, no beeps for me, i took it out of my case 2 years ago and i cant find it, anyway, i have a LED light thats supposed to light along with beeps so i was trying to detect my problem following the red light.

At first, red light was not even blinking as if everything was alright, the light was BLUE, that means everything is fine, so i tried reseting the BIOS batter by taking it out, and with the jumper. After i did that, problem persisted but now i have red blinking lights and no BLUE light at all, sometimes it gives 3 short blinks, sometimes one long, sometimes one short one long, depends on what i do..
When i take out my RAM (i have only one stick) the red light blinks 3 times, short.
When i disconnect my HDD, red light blinks one short one long.
When everything is placed back, both RAM and HDD, it blinks one long.

So i believe my motherboard is okay, right ? but no assumption yet.

I dont think its GPU problem, or CPU problem because i use CPU with integrated graphics anyway, and i have two CPU's, i tried both, nothing changes.

Should i try replacing CMOS battery, is it possible that its dead ?
Should i replace RAM ?
Should i buy a new PSU ?

The problem is... i dont have cash for all at once, i can buy battery and PSU, or battery and RAM, but i have no idea which option to choose...

I have no repair shops nowhere near me, and this is old-ish PC, im not sure if i want to drive 5 hours to get it repaired and pay probably a ridiculous price.

Any suggestion is much appreciated!
Thanks! :)