Why would turning off a fridge/freezer for 24 hours then turning it back on make it work again?

In many cases (particularly with "modern" fridges that use one set of coils in the freezer and blow air around for the freezer & fridge, rather than having a set of coils in each space and no fans blowing air) the problem root cause is frost buildup in concealed air-duct spaces or ice actually blocking a fan from running.

While this may be from a failure in the defrost system, often it's just marginal design (the defrost system is not broken, but not completely effective as designed, so frost and ice gradually build up, and there's no "broken part you can replace" to magically cure the design error.)

Taking more care to seal up things in the fridge that will add humidity to the air that will freeze out on the coils can help. Replacing the door seals if the door seals leak can help reduce the inflow of humid air from outside.

If a door was left slightly open for a while, that can cause this all by itself, due to humid air entering, and may need nothing more than "be sure the doors are closed" to "fix" it long term.

I have had a fridge in a rental that I could not get the landlord to deal with, so I'd have to put everything in a cooler and remove the back panel of the freezer about once a month in summer to clear the ice buildup (it would cause fan noise when it was getting bad) with a blow dryer (don't use an ice pick or other hard tool - poking a hole in the refrigerant system is usually going to cause a repair so expensive you might as well just replace the whole fridge.) The defrost heater clearly operated, but ice built up beyond where it reached.

I've had far fewer frost problems with "old fashioned" manual defrost fridges, but those are difficult to find new, now-a-days.

Anyway, the day of not running allowed (at least some of) the ice to melt, clearing the path for cold air movement. Or at least that's the most common scenario I'm familiar with to cause this "fix" to work.