You added or removed columns from a Mapping and hit Save. Connect has been stuck for more than 5 min in the Alter Schema state.
Being stuck in Alter Schema is usually caused by a query holding open locks on the table. You can verify is that is the case by inspecting the output of pg:locks. If there are queries that are holding locks, you can kill them using pg:kill. You will be able to identify queries generated by Connect because the application_name will have an "hc." prefix.
heroku pg:locks heroku pg:kill pid