Texmaker stopped compiling LaTeX code after I (finally) upgraded to High Sierra (btw, what a stupid name; it's not like the U.S. has a shortage of rock formations to use as names). I avoid upgrading operating systems because of this very problem: things just stop working. Fortunately this one turned out to be an easy fix.

I came across this StackOverflow question that helped me solve my problem: the LaTeX programs were not where Texmaker expected them to be. I don't know if High Sierra moved them or if a new version of Texmaker was installed as part of the upgrade. They just weren't where they were supposed to be.

I needed to figure out where the programs were, I tried to figure out where the latex and pdflatex execution files were. In Terminal, I typed

which latex
which pdflatex

I then copied the directory that resulted from each of those commands into the Commands section of Texmaker's Preferences (Cmd-,). And, miraculously it worked. And I could still get my ASA submission in on time!