Dear Susie,
From the description, it seems that you are using Excel for the web.
If yes, could you open the affected file with the Excel application and use the grouping and ungrouping features normally?
May I know whether the issue occurs in the specific file?
If it is convenient, I'd like you to share the file with us for us to do a test in our environment. If the file and the file link can be shared here directly, please add your confirmation and provide the file link here.
If no, welcome to let us know and we'll take the next step to collect the file.
To confirm whether it is a temporary issue, I suggest you fully clear the browser cache, close the browser and sign into the browser with your account to do a test to see the result.
It is appreciated if you can let us know the data range you can't group and ungroup as well.
Thanks for your effort and time again.
Sincerely
Cliff | Microsoft Community Moderator