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#11
For some reason no one suggested the obvious so far.
It is very likely that your computer actually successfully enters the BIOS, but you simply can't see it on any of your monitors, since BIOS output goes somewhere else. Which graphics subsystem is used by default before OS boots is usually controlled inside BIOS settings.
Firstly, as you yourself guessed, without a shred of doubt, the first thing to do would be to disconnect all monitors except one.
Secondly, connect that one remaining monitor to the most "basic" video connector in your system. If your motherboard has integrated video - try that one first. If it doesn't work - try other outputs one by one.