1) Make sure Outlook really is the problem!
First, you need to verify if the problem is truly with Outlook. Use an alternate method of connectivity to open your mailbox. For example, you might try accessing Outlook Web App (OWA) or through Exchange ActiveSync.If the mailbox is accessible through OWA or Exchange ActiveSync, it means the problem is likel to bey Outlook. If it isn't, the problem is then probably related to an Office 365 connectivity issue or even a corrupted mailbox.
2) Can you use Outlook from a different PC?
If you can determine that Outlook as a probable cause for a connection problem, try using Outlook to open the your mailbox on a different PC. If this works, then you've verified that the mailbox is not corrupted. If you can't open the mailbox on any PC but Outlook works for other mailboxes (but not yours), there may very well be a mailbox corruption issue at work. Before jumping to conclusions, there are other troubleshooting steps that can be taken first.3. Reset your password. It's worth a shot.
Try resetting your Office 365 password. Even though we think it should not logically make a difference, there have been a number of documented cases in which people could suddenly use Outlook to connect to their mailboxes once their Office 365 password was reset.4) Delete your profile
Try deleting your profile on the PC that you use to connect to Office 365. There are lots of documented reports of profile corruption that can interfere with Outlook connectivity. Most of these reports center on the Outlook OST (offline storage) file.If the OST file is to blame, deleting the user profile should correct the connectivity issue because the OST file is a part of the user's profile. There isn't a definitive explanation for why the OST file becomes corrupted, but some speculate it may be a result of the way Outlook 2013 compresses OST file contents.
NOTE: As a first step though, I would actually recommend that with Outlook closed, you locate rename the OST file first. Try opening Outlook to see if that helps (the OST file should recreate itself, though this may slow Outlook down a bit while it completes this task - how long depends on how much mail you have). If that doesn't work, THEN try deleting the profile.
5) Try Microsoft Online Services Sign-In Assistant
If you can't solve your Outlook connectivity problem so far, install the Microsoft Online Services Sign-In Assistant. This tool shouldn't *technically* be needed, but some find installing it corrects connection problems for Exchange Online. You can download the tool here.NOTE: Microsoft Online Services Sign-In Assistant is NOT compatible with Windows 8.
