1. This occurs when the computer was in Sleep mode , Hibernation mode, or Standby mode and then weak up.
2. Microsoft Office Communicator/Lync is asking for credentials.
1. If Office Outlook wakes up before the Microsoft Online Services Sign In client successfully signs in, Outlook will prompt you for credentials before you can successfully connect to Microsoft Exchange Online. Please enter your credentials in the dialog box. This forces the Sign In client to automatically pass the credentials and the credentials prompts no longer appear.
2. Please follow the step below to disable the automatically start of Communicator/Lync.
a. In communicator click Tools, and then click Options.
b. Uncheck the box Automatically start Communicator when I log into Windows, and then click OK.
c. Quit Communicator.
d. Quit Office Outlook.
e. Quit the Sign In client.
f. Start the Sign In client, and then sign in.
g. Start Office Outlook from the Sign In client.
h. Start Communicator.