1. There is a server issue preventing login. 
2. There are general connectivity issues between your machine and The Elder Scrolls Online service
3. When your game client is out of date.

First, check the Service Alerts  and Twitter Support pages to see if there is a current outage. If there is, the error will persist until it's resolved. 

If the servers are not indicating an issue, proceed to the following troubleshooting steps: 

• Ensure the game is properly updated by closing and re-opening the ESO launcher.  
• Repair the game files by following the instructions found here.
• Restart your computer.
• Add the Zenimax Online folder as an antivirus exception.
• Forward the appropriate game ports found here.

If the problem persists, contact your Internet Service Provider for additional assistance.