Hello,
I'm using Debian Jessie and have managed to break it - X won't start when I boot up and I can only use the command line. When I type startx the following is returned:
/usr/bin/X: error while loading shared libraries: libpng16.so.16: cannot open shared object file: No such file or directory.

To give some context, before this happened I downloaded and installed the latest libpng libraries from here:
http://www.libpng.org/pub/png/libpng.html

I think, however, libpng was already installed. When I navigate to /usr/local/lib and view the contents, they include:
libpng16.a
libpng16.la
libpng16.so
libpng16.so.16
libpng16.so.16.16.0
libpng.a
libpng.la
libpng.so

the three libpng files (without the 16) are links to their respective libpng16 files. libpng16.so and libpng16.so.16 are links to libpng16.so.16.16.0. And the files libpng16.a, libpng16.la and libpng16.so.16.16.0 are not links. All of these files have permissions rwxrwxrwx (777).

I have not been able to locate the config and log files for xorg (I did try the find command but it didn't return anything).

Does anyone have any advice? Thank you for your time.

UPDATE: Nevermind, I ran "sudo ldconfig" and it's fine now. I did search before writing this up and that suggestion had slipped me by, honest!

The way to know what's going on is to run these

then run file on the target of symlinks. Most systems need 64 bit files these days. Debian uses lib32, but most others put 32 bit in /usr/lib32 and 64 bit in /usr/lib64. If you have a /usr/lib64 with just a few files, that is probably wrongly installed. Applications need libraries of the correct width; 32 bit apps need 32 bit libs, 64 bit apps need 64 bit libs.