Never seen an app get deleted by exporting it so :-( that stinks
1) if its not backed up somewhere and it really is deleted its gone
2) if its in another environment restore it from there
3) go to the maker portal and click Apps and select All, do not go into your solution
or
3b) go into your solution, click Apps tab, click Add Existing and try to add it back to your solution, if you do not see it at first, make sure to click on Outside dataverse in the Add existing
Not sure what else happened :-( but that's what you can do to restore it, if its there.
Getting it from another environment and back loading it can work too.
If these suggestions help resolve your issue, Please consider Marking the answer as such and also maybe a like.
Thank you!
Sincerely, Michael Gernaey