Are all keywords gone from the photos, or only some? Are the most recently added keywords gone, or also keywords you added years ago?
Photos may have opened an older version of your Photos Library instead of your current library. This can happen, if Photos has a permissions problem with the location of the Photos Library. Try to open the correct library directly by double-clicking your Photos Library. You can also hold down the options key while launching Photos. Keep holding down ⌥, until you are seeing the Library Chooser dialog. Is there more than one library shown?
Mojave is more restrictive with the access to Photo Libraries because of the added security features. Is your Photos Library on an external drive? Then check, if the drive is suitable: Move your Photos library to save space on your Mac - Apple Support