I deleted a lesson by the “trash icon button” and now I can’t see it again on the library.
How to get the lesson back to the library list ?
The lesson should still be in the Library. Try using the Advanced Search, but make sure the “Don’t show previously selected lessons” box isn’t checked 
I founded where you say it. But at the default library list, the lesson will not appears again ?
That is correct. The default Library page will only display lessons you have never taken.
If you want to remove lessons but still access them later, the “Archive” function might work better.
Ok, thanks for the answer.