I have a single operating system (Ubuntu). When I start my computer, I get a black screen, and after a few seconds, the Ubuntu loading screen, and after that I'm able to use my computer as usual. I do not get the BIOS loading screen. I've tried F2 and F12 (which are the keys used in Toshiba Satellite, my laptop) while the black screen shows to enter BIOS setup, but those don't work.
How do I fix this? I've already reinstalled Ubuntu (not because of this) and the problem still persists, though I doubt that Ubuntu is the problem.
EDIT: My questions is not how to enter the BIOS setup – I know how to do that. The problem is that I don't even get the usual loading screen that says "TOSHIBA" and "Press F12 to enter setup". My computer, on being turned on, directly shows the Ubuntu loading screen
You may have inadvertantly selected a "quick boot", "boot logo" or similar setting, which replaces the BIOS' normal output with a graphical image and might skip certain checks to make the system boot faster. But I find it hard to see that something like that would have changed without you actively doing anything to the BIOS.
Removing the CMOS battery, or using a "CLEAR CMOS" jumper on the motherboard (at least on full-sized motherboards there is usually one that can be set to either "normal" or "clear"/"reset"), would be the first thing I try. You'll lose all BIOS settings, including the system date and time, but once you get into the BIOS setup a "restore to factory defaults" or similar should be all that is needed to bring the settings to reasonable values. Remember to reset the system time afterwards, preferably before booting (some things may be upset when the system time makes sudden jumps of many years). Note that since you have a single-boot Linux system the OS may be configured to read the system clock as UTC rather than local time.
Clearing the CMOS memory by any means will usually force you to enter the BIOS setup on the next boot, because then the internal data checksum won't match.