hello Jean,
that is a normal situation in apple’s Genius Bar. They can’t repair your tablet because they don’t know what is wrong with it. A fault suggest hardware issue but it can be a different thing. Let’s find out together :-)
Firstly, important thing is its history. What happened with it: any repair, drop or somethings. Secondly, capacity and last copy of your data. Then thirdly, iCloud and its usefulness.
Is your tablet connects to pc/Mac? If yes, restore it with no copy. Install your iCloud account but don’t restore a last copy. Use a fresh install. All data will actualizing via iCloud account. If tablet starts normally, you will be able to install all your apps. That all for first step. Let me know :-) take care,
* regards,