There's a boot key press combination to interrupt normal start up and select an alternative boot medium. Usually things like f12, f1, del, or esc, mashed at the right time after power-on.

For locked down devices like modern smartphones and probably games consoles etc, it's a lot more involved, and it may be next to impossible to boot a different OS, but there's often a combo to get at least into a factory reset boot menu.

Edit:
lookinto 'system-rescue-cd' or 'knoppix' which are pretty useful USBs to have around lots of tools to try to mount and recover disks and move data . 'clonezilla' can also be useful to create a disk image before you try any risky steps.

Beware Modern windows likes to encrypt which might impede those tools.
I've seen cases where the user doesn't know (claims to never have been told!) the decryption key.
In those cases recovery may be hard/complex in some scenarios.

But a proper backup system (3-2-1 , fairly often) for important data is a far better way to prepare for "shit". And "the cloud is not a complete backup system" is not a bad a general rule.