turning on pc after freezing twice suddenly there's no display.

may help reaply thermalpaste between fan and cpu.

Overheating problem? Maybe try to re-seat the videocard. It could be related to the power supply as well. Also try removing and plugging in all the connectors. If you have any canned air, maybe blasting some through the cpu heatsink might help too. There could be lots of causes for this, but we would need all the details you could provide to be more helpful.

Well, we narrowed down that it's not likely the PSU, cables, ect. as you swapped them out with other working models. Just to check, did this PC work properly at all since you installed the 2nd hand motherboard? Does your motherboard have onboard video, or do you have a standard videocard installed. I would try a spare card if you can borrow one or have one laying around. Also, are you sure your computer is actually posting when your turning it on? Does your motherboard have the little speaker built-in to hear some beep codes?? Another way to check, is does the cpu fan speed change at all after it's turned on for a few seconds? Usually computers will run the fan at full speed until POST is completed when they're first turned on before lowering speed based on thermals until a higher speed would be needed. If the speed/volume lowers, then your computer would be passing POST. If it stays constant, it may not be passing post. I'm not sure however if your cpu fan is a 3-pin or a 4-pin PWM fan. If it's a 3 pin, it probably can't speed down as that's a pretty modern ability for non OEM PCs. Short version is if it's a 3-pin, this fan speed check may not apply. Also, what is your motherboard brand and model #?? Also, check for any leds lighting up on your motherboard in specific areas when it's turned on, they may provide a hint to the problem. One more idea to try, is maybe removing and reinstalling the CPU and make sure you got the right amount of thermal compound(pea size in the middle or even spread) on the CPU and check that you got the Heatsink/Fan mounted good. Keep us updated so hopefully we can try to figure this out.

Running out of ideas, but have some more. Hopefully someone else will chime in too. I know I mentioned the overheating possibility, but probably just about all CPUs made in the last decade will simply power down the entire system if it was getting to hot. This is to prevent damage from occuring. Actually had a laptop do this a week ago that I was working on, so I wouldn't go running out to buy thermal compound just yet. However, it sounds like your system is staying on, just no display. You mentioned your cpu is constantly changing speed on power up. Do you mean it's ramping up then ramping down over and over, or just going from a high speed then settling on a lower speed? How long have you left the system on waiting for the display to come up? Since you connected to your TV, i'm guessing you used an hdmi connection. Any luck with VGA or DVI?? Also i see you took the battery out, did you also just moving the CMOS jumper as well(the red jumper on the bottom right side of the motherboard if your looking at it mounted in the case). By default it's on pin 1-2, with system powered down, move to pin 2-3 for a bit, then return back to 1-2. You could try different ram if available, but it's sounding more and more like something got damaged somehow over that past 2 weeks. The squeeling noise could be coming from inductor coils, which are on the motherboard itself, inside power supplies, and on the video card. Is the sound there if only using onboard video vs using the card? Since it makes it with both PSUs, it probably not that, if it does it with onboard video, it's most likely the motherboard. Either way, the sound itself doesn't mean something won't work probably, just not sound the best(I believe it's actually vibrations in the parts themselves from electric current going through them), but this may actually be a different sound than your actually referring to. Kinda getting off the big picture though, is that it worked for two weeks before all this happened. Do you know what was wrong with the first motherboard before you replaced it? It seems like something slowly got damaged over the last two weeks. A long shot here, but maybe the old PSU is what killed the old motherboard, and ended up damaging the replacement motherboard that you put in two weeks ago. What brand/model power supply is the old one?? If it had something wrong with it and sending an out of spec voltage to the motherboard, it could have damaged it that way.