A charger showing offline in the app can be either as a result of the charger not receiving power or a communucations issue.
To verify that the charger is receiving power, check the charger to see if any lights on the charger are on. If the charger has a display check that the display is on.
If charger lights/display are off, the charger is likely not receiving power. Learn how to resolve power issues with your charger.
If charger lights/display are on, than the charger is receiving power and this is likely a communications problem. Try to restart your charger. If the problem persists, please open a ticket.