HI CP,
Try these steps:
With the computer started and macOS fully launched, and no other applications open:
Press and hold the shift key as you launch Pages. Keep the key down until the launch completes.
The shift-down launch tells Pages to not load the items previously open and to not restore the previous state.
With Pages running, try creating a new document from a template in the Template Chooser.
If successful, try opening a saved and editing document that has not given you previous trouble.
If the issue persists, Relaunch the computer into Safe mode.
Note that this launch is slightly different for computers running on Apple Silicon than for those whose cpu is an Intel chip. Details on Safe Mode and the starting sequence are here: How to use safe mode on your Mac - Apple Support
Regards,
Barry