Hi @Lorrine ,
Welcome to our forum!
Does this issue occur in Web mail?
And have you done any specific operations before that?
If the issue only occurs in Outlook client, considering several factors (such as corrupted data file/Outlook profile, disconnected network, damaged Outlook program. etc.), please refer the following article to troubleshooting:
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.