If at some point when working with ArcMap, the symbols for points, lines or polygons cannot be correctly seen. Then, there is likely to be a conflict in the use of fonts installed on Windows.
The Symbol Selector window requires the fonts installed in the system to structure the symbols. If one is missing or unreadable, the system will automatically search for a substitute from a library of available fonts.
Generally, the ESRI Cartography font (esri_1. ttf) is the most frequently damaged and therefore the symbols are shown broken. To solve this problem it is necessary to replace it with a functional one. Before you start, don’t forget to back up the fonts you are going to modify.
You must first close any application that is in use. Then go to C:\Windows\Fonts remove the damaged font (or all that say ESRI) and replace it with one of the ESRI fonts in good condition. Open ArcMap and the problem must be solved, another option would be to reinstall ArcGIS, but that takes a long time.
This trick has been tested in ArcGIS 10.2,10.3,10.4 and 10.5.