Are the window openings showing (empty) in Elevations, or nothing at all? If empty openings, check Model View Options setting for the relevant elevation views. If nothing at all, try checking for duplicate walls.
Check you Door/Window-related Model View Options.
Doors and Windows can be set there so that only the Opening is shown.
You may have activated a View that uses such stored settings and this has caused them to disappear.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD 2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28
I just had the same problem and it turned out even though it looked like I could see the wall in elevation, I couldn't really. The wall was mistakenly on an internal wall layer and my elevations are set not to show any internal walls.