How old is it? Is it still under warranty? Have you tried a video reset? Boot up the PS3 by holding down the power button. Continue holding until you hear a beep. Release the button after the beep and let us know what you see.
I remember this happened to me once, with 3.50 Downgrade... I didnt quite understand the service mode jig... I found out my problem was I had both video (AV) and HDMI hooked up). After booting the jig it would just give me a black screen and then turn off after so much seconds...
I pulled out the AV cable because i had already done the jig with HDMI..., and then do a couple of hard reset's (Holding down the Power button like suggested above) Then eventually it came back on...
I've recently recieved many inquiries about problems occurring after people have attempted to repair their own PS3 systems. The most common problem I keep hearing about is that after successfully restoring their PS3's functionality, they notice that the fan speed seems to rev or is constantly on high, moreso than before repairing the unit/s.
I have isolated that the most common reason for this is heat sink/thermal pad damage, loss, or offset placement upon reassembling the board with its case. There are numbers of little pads for various chips on the main board in different arrangements depending on the model. The heat sink pads can easily fall right off and many people don't even realize that they've lost them.
Also, during a heatgun reflow, many people are damaging pads that should be removed before heat is applied, creating holes, etc, having a similar effect to not having the pad/s at all. These pads are vital for the system to function correctly, maintaining constant temperature so as not to YLOD again. As many have found out, the seemingly repaired PS3 unit will always fail if these pads are not placed correctly.
The key indicator for incorrect pad placement is the obvious, quickly increasing fan speed, as the system is trying to compensate for bad heat sink contacts, leading to various overheating issues which sensors are reporting.
I don't recommend attempting a PS3 repair to anyone without the proper understanding of internal computer circuitry. Murphy's Law always seems to make things worse when the proper attention is not given to vital elements of such a complex machine.
actually for me the most common problem I see is they've bent the copper cooling pipes holding the heat-sinks in place (thermal paste sometimes puts up a fight I guess) so the pressure isn't applied evenly correctly when the board is reassembled.