When a terminal is in Service Mode, it usually indicates that the terminal has an incorrect configuration and is running in safe mode. The terminal will say "Terminal in Service Mode" on the screen.
The first step should be to reboot the terminal. If the terminal launches back into the service mode or starts to boot loop again, try to access the admin menu to sync or remove the configuration. Trying to update the configuration or re-boarding the terminal should be the first steps you try.
If it's not possible at all to access the admin menu and the terminal continues to boot loop or fall into the service mode menu, the terminal will need to be replaced.