Once a project has been deleted from the workspace, it cannot be undeleted. This is a reason why Smartcat asks for a confirmation before proceeding with the deletion.
However, even if it is not possible to undelete the project, the translation that has been completed up to date is still stored in the translation memory so no work is lost.
The easiest solution is to simply recreate a new project using the same source files, associate the TM from the deleted project and set pre-translation rules to pre-populate the target segments with the previous translation. If all the segments were translated and confirmed, the work will be recovered and the target document can be generated.