My car died while I was driving it. I have a 2002 Nissan Altima. Was driving about 20 miles per hour. I was able to start it again, but had to press on the gas pedal a little, although for a few seconds I thought it was not going to start.

The first thing to come to mind would be a clog of something, some kind, in the gasline.
You were, I think, fortunate that it happened at 20 m.p.h. if it happened when on the highway it cut out when you were going 60 m.p.h. it could spell disaster if the car died on you and approaching traffic behind you had no place to go with their speed constant.

Get the car checked by a qualified technician... probably best at a Nissan dealer.
It may be nothing but an electrical connection which can be easily corrected... or it could be something more involved.

If you have had the car longer than... say... 2 - 3 years, the fuel pump may be needing replacement.