iMessage shows error message: "Not Delivered"
You probably somehow got disconnected from iCloud. First, you should probably log in at icloud.com with the Apple ID that you normally use for the Messages app to ensure you're not having a general account issue. If that's successful, while using the Messages app, select Preferences from the Messages menu. Click Accounts and select the iCloud user that appears for you. Be sure Enabled has a check mark next to it. If it does, remove the check mark and use the provided fields in the window to log back in. If you don't see Enabled, use the provided fields to log into your iCloud account. Once successfully logged in be sure to put a check mark in all the boxes that apply.