I spent some time looking at youtube videos on adjusting derailers and adjusting the three screws. The main shifter stopped being able to shift and I am not seeing anything specific that could be causing this. The cable is intact and I don't think I have damaged anything. I wasn't able to take a ride today because of it.

Disconnect the cable from the RD and remove the cable and housing from the RD.
Pull some shift cable at the lever so there is slack there.
Now manually operate the RD while cranking the pedal. Does it move freely small to large and back. If not then why? Rusted up? Damaged somewhere?
Operate the shift lever. Full travel, smooth and easy. If not then why?
Now use the lever to operate the cable through each housing section, one at a time. Full travel, smooth and easy? Slide each housing section down the cable a ways to expose the cable. Rusted? I see a lot of cables that look fine from 5 feet away but are all rusted inside the housing. If any rust then change cable and all housing for that run.

I would not be surprised if the problem was a rusted cable and housing liner. Don’t try to save it. Change it.

When you don't know what you're doing...DON'T...
Watch more videos...keep trying...

I kept adjusting the screws, used some WD40 on the cable, it is shifting ok now. Not perfect but better.

What type of shifters? Usually trouble is a gunked up derailleurs, but shifters especially the Suntour barend can be troublesome.

Suntour, Motebecane, mid 1970s. vg-luxe. Generally seems ok. If it's not broken don't fix it is my advice. Lesson learned on using gas station air, and WD 40.
Garaged, so bike has little rust.