what happens during a restart?

Big question! Could write a book on that, but many others already have... For a very short answer, shutting down Windows (hopefully using CTL-ALT-DEL and selecting shutdown, but if no choice then the off button) and turning off the computer clearsvolatile RAM, so any corrupted code is erased. Power off and then back on resets most hardware devices, so the Application Program Interface (API) gets a fresh stary when the drivers are reloaded. And, Windows pointers, counters, and branch tables(AKA "resources") all get reset, so Windows itself is starting clean. There is much more to it, but in my opinion those are the major points.