Looks like there was an automatic firmware update. If yes, you should reconfigure the router.
Some settings go back to Default, enable Allow guest access.
You will then automatically receive a password for your guests.
That also happens here.
If you haven't set up automatic firmware update, you may need to update the firmware.
See the following Apple articles:
Recommended settings for Wi-Fi routers and access points
Recommended settings for Wi-Fi routers and access points - Apple Support
If you need help with your Wi‑Fi password
If you need help with your Wi‑Fi password - Apple Support
if you have a Mac use IP Scanner Home app to sign in to the router, see:
https://apps.apple.com/nl/app/ip-scanner/id404167149?mt=12