I opened the game today and all of my installed mods are gone. They no longer appear in the mod tab nor are they loading. Does anyone have any idea what would have caused this and how I can fix it.

Best guess is the game updated and decided to "repair" setting all files to their "intended" default state deleting all your mods in the process.

To fix redownload and place mods in the correct locations (usually the mod folder for most mods) and disable automatic updates in GOG Galaxy.