Hi there!
I am a complete beginner with glyphs so I am sorry in advance if my question will sound a bit vague.
I used to create a new stylistic version of my letter and after it was finished I clicked on Text (T) to type the original letter near it to see them together. After that I wanted to delete it but accidentally clicked on delete 2 times instead of 1 and my new letter disappeared completely. Is there are any ways to restore it? Please helpðŸ˜
Tried with revert/command + z, nothing is working but maybe there is a way
Is it not present in Font view anymore? What happens if you search for it with Cmd+F when in Text mode?
not showing it anywhere anymore 
Do you have any backup of your file? Sorry to hear you lost your progress, but this might act as a good lesson. Save your file often and find some kind of versioning system that works for you (git might be a bit overkill, but it works really well).
thank you, will be more careful next time!
In order to delete a glyph, you need to Cmd+Delete, and confirm a dialog. If you did not confirm that dialog, it is not deleted.
Look in the Font tab (Cmd-Opt-1, make sure you have no filter or search string in the left sidebar) and perhaps in backup layers of the glyphs.