Have you tried switching Fly mode (F key)?
Also if you Page Up do you just fall down again?
There are other posts here that talk about the navigation keys not working.
This may not be the same problem.
Seems to affect Macs.
Reboot solves temporarily but an OS update seems to fix it.
Are you running the latest OS?
https://archicad-talk.graphisoft.com/viewtopic.php?f=13&t=51787
https://archicad-talk.graphisoft.com/viewtopic.php?f=20&t=50351
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