I have been using LaTeX for a long time now. It has suddenly stopped working:

`amsmath.sty not found`

I downloaded that and similar files to the same directory. Now it gives the error

`begin{document} not found`

But the files are there!

-
Welcome to TeX.SE. This problem is probably due to some other error, so it would be helpful if you composed a fully compilable MWE including `\documentclass` and the appropriate packages that reproduces the problem for you. –  Peter Grill Feb 23 at 19:43
If you use MiKTeX, your problem may be explained in the following post: tex.stackexchange.com/q/96778/19384 –  Peter Jansson Feb 23 at 20:02
If nothing else works, you can always make a new installation, but this is og course as a last resource. –  Svend Tveskæg Apr 4 at 12:57
If you use TeX Live, does `kpsewhere amsmath.sty` in a terminal window print out the path to amsmath.sty? A MWE of your code would be nice to find out, if there's another source for this error message. –  Stephan Lukasczyk Apr 4 at 15:12