MIcrosoft Outlook stopped working on Yosemite

After to Alistair's comment, I did some more general googling. I found some suggestions that the best solution was to rebuild the identity, so that's what I did:

1. go to ~/Documents/Microsoft User Data
2. make a "backup folder"
3. drag the contents of ~/Documents/Microsoft User Data/Office 2011 Identities into the backup folder
4. relaunch Outlook and configure as if it's a fresh install.

This has things working again.

Without more clues its hard to give specific advice, but this is what I would try.

Clear out the Saved Application State

Look for Outlook's saved application state folder within your account. You should find it at /Users/YOURUSERNAME/Library/Saved Application State/com.microsoft.Outlook.savedState. Just rename or delete the folder. Then launch Outlook to see if it opens successfully.

Hide your Outlook Identities folder

Rename the /Users/YOURUSERNAME/Documents/Microsoft User Data/Office 2011 Identities' folder to something likeOffice 2011 Identities Old`, and try Outlook again.

Hide Outlook's preference list files

Within /Users/YOURUSERNAME/Library/Preferences you should find four Outlook plist files.

com.microsoft.outlook.database_daemon.plist
com.microsoft.outlook.databasedaemon.plist
com.microsoft.outlook.office_reminders.plist
com.microsoft.Outlook.plist

Rename them or drag them to another location temporarily, and try Outlook again.

There are also Outlook plist files in /Library/Preferences/. On my Mac they are blank, if they have content on your Mac you might want to try renaming or moving them temporarily and trying again.

Hide Office's Cache folder

Temporarily rename the /Users/YOURUSERNAME/Library/Caches/com.microsoft.Outlook folder something like com.microsoft.Outlook.old and try Outlook again.

Hide the Office Application Support folder

Similarly temporarily rename the /Users/YOURUSERNAME/Library/Application Support/Microsoft/Office and try again.

Still not working?

Try running sudo opensnoop | grep Outlook in a Terminal window and see if you can work out which files Outlook accesses before giving up and closing.