I've been working on this source file for a while, and indenting it just fine, and now suddenly I cannot indent lines by any means (tab, M-x indent-region, etc). Restarting Emacs doesn't fix it, closing and reopening the file doesn't fix it, and the problem seems to be isolated to this particular file.
It's a cpp file if that matters. Yes, I am working in C++ mode. Yes, syntactic indentation is on.
Look at the last part of the file that you edited. Most likely Emacs is in fact indenting, but your source code is such that indenting at the position where you are trying to indent has no effect.
IOW, look at code that precedes the position where you are trying to indent. Look for syntax problems that might be making that position actually appear to be top-level. My guess is that a syntax problem is throwing off the indenting.
You can also narrow the buffer, using C-x n n, to particular parts of the buffer, to see just where indenting does what you expect and where it does not. That will help you find any syntax problems that might be interfering.