I haven't had this problem for some years but previously I have come across something similar.
I can't say this will work for sure but it sometimes did.
Open a file based on the same initial template as the one you can't open.
Now don't close this file but go to FILE > OPEN and select the file you can't open.
Make sure you do not select the "Start new instance of Archicad" option.
Open the file - of course the original file will need to close but this sometimes seems to reset some of the missing information.
If that doesn't work then all I can suggest is trying to merge the damaged file into a good one - again based on the same initial template if you can.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11