first here is the fix:
Take your nexus 5 out of it's case (if it has a protective case). Turn it around so you face the back.
Start pressing the cover back on to the phone on the top right area (a little towards the middle of the phone) - until you here a click! - that's it - fixed.
why this worked:
There are small plastic clipses on the inside of the cover that attach the cover to the phone - they run around the rim of the phone and in the middle.
The charging unit is attached to the back plastic of the phone and it has small metal contacts that touch the main board in different sections.
Sometimes when the phone falls, the back separates a little from the phone (the middle plastic internal clipses) and then the back charging unit looses contact with the phone.
