How to reinstall windows Win10 on a Dell??

Hello,

I brought my first Dell device a Desktop PC in July this year. It was running perfectly until this morning when I turned it on but Mozilla Firefox Browser wouldn't open. I had been sing Chrome for over an hour and it was working fine. So I did a re-start, it booted, but got stuck saying "Restarting" with the circle going around but nothing more happened for nearly minutes so I turned it off at the wall.  Tried it again but same thing happened so I tuned it off at the wall again and left it that way.

Meanwhile I hunted the entire place house looking for the Dell Box with paperwork, discs cables etc but I cant find it anywhere, which is very odd as i am always very organised and have kept everything PC related, in fact I still have all my old PC parts etc from 1998.   Presuming that I would have to reinstall windows like I'ce done a zillion times before with old PCs was no problem can do it with my eyes closed except..... I had NO windows disc!!!

Trying not to let the panic set in,  I turned it back on at the wall and the Tower, and to my great relief it started normally. The first thing I did was log into my Dell account but became concerned when in my account details areas, there was no mention at all of my purchase of this new PC back in July. 

My question: ... How do i reinstall windows on this Dell???

I know, its embarrassing, having to ask such a basic task...  but would appreciate some help please.

Many thanks
Helen