Sending, receiving, and organizing email in Outlook.com
Dear user,
Welcome to the Microsoft Community.
For Outlook.com:
You need to log in to the outlook.live.com first, then remove at least 10 MB of attachments to unlock to update the storage quota (regardless of whether or not the attachment needs to be retained), once this is done, refresh your browser, you may need to wait at least 10 minutes for the update to take place and, then restore the deleted attachments via the link: Recover and restore deleted items in Outlook - Microsoft Support
For OneDrive.com:
If the above scenario fails or does not work in operation, first log in to onedrive.live.com, then you need to delete at least 150MB of files to upgrade the storage quota, once done, refresh your browser, you may need to wait at least 10 minutes for the storage upgrade to take place. And then restore your deleted attachments via the link: Restore deleted files or folders in OneDrive - Microsoft Support
If the problem persists, contact Microsoft Online Support. Contact - Microsoft Support
We sincerely appreciate your cooperation and patience in this matter.
Best regards
Mia.W - MSFT | Microsoft Community Support Specialist