I would get in touch with Support and log a ticket ASAP. Perhaps there's something they can do to reverse it.
Otherwise, if Marketo is hooked into a CRM system, you could always sync email addresses back over from your CRM or export/import a list and upload it that way.
In the worst case scenario, the old email address is still stored in the Activity Log and can be viewed there - so there's still a record of it in the System. That's not a feasible way to update back in bulk, however.