If LaTeX finds an error in your document then it gives you an error
message and prompts you with a question mark, ?. For instance,
running LaTeX on this file
\newcommand{\NP}{\ensuremath{\textbf{NP}}}
The \PN{} problem is a million dollar one.
causes it show this, and wait for input.
! Undefined control sequence.
l.5 The \PN
{} problem is a million dollar one.
?
The simplest thing is to enter x and RETURN and fix the typo. You could instead enter ? and RETURN to see other options.
There are two other error scenarios. The first is that you forgot to
include the \end{document} or misspelled it. In this case
LaTeX gives you a ‘*’ prompt. You can get back to the command
line by typing \stop and RETURN; this command does its
best to exit LaTeX immediately, whatever state it may be in.
The last scenario is that you mistyped the filename. For instance,
instead of pdflatex test you might type pdflatex tste.
! I can't find file `tste'.
<*> tste
(Press Enter to retry, or Control-D to exit)
Please type another input file name:
The simplest thing is to enter CTRL d (holding the Control and d keys down at the same time), and then retype the correct command line.