I assume you are trying access an Outlook email account using a browser opened to outlook.com. If this is not the case, please provide some additional details about your situation.
Based on these post, it looks like it might be a browser/cache/cookies problem. Try a different browser (e.g. Edge), clear your cache and all your cookies, refresh the web page.
https://www.reddit.com/r/Outlook/comments/1264yb8/help_my_outlookcom_inbox_wont_load/
https://www.reddit.com/r/Outlook/comments/1h6ohp5/help_i_cant_open_my_email_it_gives_me_this/