When I load a buttons pack, all my buttons disappear. Why is that?...

This could occur for a variety of reasons.

Most likely is that the image files you are trying to load for the buttons are in the PNG format, and doing this requires 2 files to be present in your foobar2000 main directory. These files do not come standard with the program, but are freely available.

You will need:

libpng13.dll
zlib1.dll

Hopefully, this works for you. Note, do not put them in the 'components' directory; make sure they are in the main foobar2000 directory.