Hi markfromroermond,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with opening a spreadsheet with Numbers. We're happy to help.
When attempting to view the spreadsheet are you using a supported web browser? If needed, you can find the supported list here: Use iCloud with Numbers on Mac
Also, is this a shared and password protected spreadsheet? If so, we recommend checking with your colleagues to ensure the password wasn't changed. The guide here can provide some insight on password protected spreadsheets: Password-protect a Numbers spreadsheet on Mac
If the issue continues, confirm the OS you're attempting to access the spreadsheet on the iCloud website from. Also, let us know if you can test opening the spreadsheet on a different computer or with the Numbers app on an Apple device.
Cheers!