You have probably switched the two dumps of the playstations and flashed the wrong one if the safe menu also does not work. This will lead to a kind of GLOD, where the PS3 stays on and green but does not output any display.
At least worth a shot. And if you downgrade more playstations, try to save every dump with the serial number of each playstation so you can keep them apart (at least, that's what I do). Good luck and let me know the result.
And for the YLOD one, there probably has been a problem during flashing. In the future, always check the E3 log on your computer before turning the playstation off. That way you still have the ability to reflash. For this one, the tristate has to be soldered to the ground, then be reflashed, and then you can remove the wire.