Here are some common reasons/solutions as to why this may be happening:
A) You may have placed a map, mesh, preset file, etc. in the wrong place. Try double checking everything is where it is supposed to be.
B) Sometimes certain folders act up at times (my particles folder does this a lot.). Try unloading files you may have put something in (particles, objects, my_objects, exports, presets, my_presets, and so on.
C) You may need to download the latest patch. Do you have it?
D) You may have deleted an important file by accident. The easiest way to get it back is re- download FH. (Note: ALWAYS back up important files, such as add ons and such.)
Try all of these, good luck!