this isnt really a legal question your projects are still on your hard drive so its not as if someone stole them or something. the simplest way to have them added back to the library tab is to just go to the location that they were stored and open the project file.
I recently rebuilt my computer with some new components and after re-DLing UE4 it shows I have zero projects saved and yet last time I logged in I had 5 different projects saved with well over 100 hours worth of work put into 3 out of the 5.
Where are my projects and why were they not saved its proprietary work and I both want and am legally entitled to get it back