Are you due a Federal refund? If so, there is no penalty for a late Federal return when due a Federal refund.
If you owe taxes due, however, and if you did not request an extension, and if you originally efiled on or prior to April 15, then you have until tomorrow April 20 to either successfully efile or to print, sign, mail, and postmark the return in order to be considered timely-filed. NOTE: If you end up mailing, be aware that some post offices close early on Saturdays; some close at Noon, so be aware of final mailing/postmark times in your area.
The deadline to successfully re-efile or mail is April 22 in Maine and Massachusetts due to Patriots Day.