MaridAudran: The exact model number is:
Satellite C855-S5355 Serial No.
9C245654Q Here's something I found Googling that model number. It seems like according to that you may have to press F2 after it's powered on and while the TOSHIBA logo is displayed?
Judging by the third post on that topic, that appears to have worked for that person with the exact same model number, so if you can't access the BIOS (or 'UEFI Setup Menu', apparently) with that technique, it would seem something else is wrong with the laptop. I know that sometimes holding the FN key on laptops allows the F1-F12 keys to perform alternate functions, so you could see if the FN key is stuck and preventing the normal F2 operation.
And as NoNewTaleToTell says, you could try tapping F2 rapidly while the logo is onscreen. I usually end up doing that whenever trying to enter BIOS since sometimes it seems you have to hit the key at just the right moment for it to catch it.
Oh, and
here's a page for a slightly different model number that seems like it may be relevant: it discusses downgrading to Windows 7 from Windows 8 and describes some issues accessing BIOS as a result.
Now after Windows 7 have been installed on my system, I ran into an issue. When Windows is loading, my F2 key does not take me back into BIOS so that I can make changes. To be Frank, none of the keys work at this point. Don’t worry, I have done all the work for you.
To enter into the bios just follow these steps:
1. Turn computer off
2. Now press and hold the F2 key
3. With the F2 key still press, press the power button
4. When the system lights turn on, release the F2 key.
5. Now you may sometimes see the Toshiba Logo, or you may not. The system will still enter into the BIOS.
Note: If the key is held too long the system will make a long beeeeeeeeep….not to worry it will still enter into the BIOS.
I don't know if that is the same procedure you tried earlier or not and I know that the guide earlier describes changing boot mode from UEFI to CSM Boot before downgrading to Windows 7, so I don't know if that has to be done before those steps work. It is bizarre to me that a laptop would be dependent on Windows 8 to access the BIOS...