...what that bottom bar where Start, etc., is called

Task bar. Although maybe it's the tool bar; I can't keep them straight. All these bars are enough to drive you to drink

And if you move the mouse pointer (A.K.A. the "cursor" - lots of room for expressiveness there) precisely onto the boundary between the bar and the rest of the screen, you get a little double-ended arrow, and then you can click-and-hold on it and, while holding it, drag it to make the bar bigger or smaller or even move it to the top or the right side or the left side. If you've a mind to.

And if you think you've lost it, you probably didn't, but it's been reduced _so_ small that it's only a little line at the edge of the screen. You can get it back again by moving the cursor all the way out to the edge - very precisely, remember - and get that double-arrow back, on which you can then click-and-hold again and drag the taskbar back to visible size...