Hi @Keale
Welcome to our forum!
For better research, please provide more information about this case:
Is your account a Microsoft organization account?
Do other users in the organization have the same problem?
Did you do anything when this issue occurred?
There are many reasons for this problem, please try here are a few possible solutions:
- Try using InPrivate mode in a different browser to sign in to Outlook Web App.
- Check your internet connection and make sure it's stable and fast.
- Please contact your organization's administrator to check if there is a setting in place to prevent users from signing in. To check this, your global admin can sign into Microsoft 365 admin center > Users > Active users > select and click on your account > Account > Roles.
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