Maintenance for the week of February 16:
• [COMPLETE] ESO Store and Account System for maintenance – February 19, 9:00AM EST (14:00 UTC) - 6:00PM EST (23:00 UTC)
You could be running into a connection issue preventing you from logging into ESO. If you continue to receive the message "Unable to connect to game server" when logging in you can go through the troubleshooting steps in the help article linked below.