Is there a way to undo a quick publish from infopath to SharePoint?
I have modified an existing published SharePoint form and the form no longer appears usable.
Can a reset be done without loosing existing data from original form?
No it is not possible. You should treat your InfoPath templates as source code and check in changes to a source control system. You should also at least have one other farm for testing.
In case something breaks, and it will, you can revert back to last version.