Before you started amending you need to make a copy of your return and work in the copy so you don't lose the original return. See if you have another .tax2018 file on your computer. To make a copy go up to File-Save As.
Did you hopefully save your original filed return as a pdf file? You can use that to help you recreate the original return. If you do recreate the original return, go all the way to the end and say you will print and mail it. Then print it out so it thinks it was filed. Then you can amend properly.
