I have no definite answer. However, some Windows users did report similar problem that occur irregularly, particularly with Asian fonts. So this may be due to a bug in Windows. I suspect sometimes when Windows do not shut down cleanly or with disk caching enabled, the font cache may become corrupted.
For me, my case was the Asian fonts were messed up, so file names with CJK characters showed up as square blocks in Windows Explorer.
If rebuilding the font cache doesn't work, maybe you can try reinstalling your software or repairing Windows. It might be using or depending on certain fonts installed in your system, which unfortunately could have been uninstalled? Reinstalling the required missing font may also help.