Hi! This usually happens due to lack of storage.
Go to https://www.microsoft.com/en-us/microsoft-365/onedrive/online-cloud-storage > Log in with your email account > Under Storage, click the blue letter > see if the storage is full.
- If you use any Outlook Desktop app, try using Outlook Web https://outlook.live.com/mail/
- If you use an email from a domain other than Microsoft, you should contact the domain's Support.