Could you please let us know which Operating System are you working on ?
Please go to the following location : C:\Users\User name\AppData\Roaming\Adobe\Lightroom
In there you should see a Folder by the name of Export Presets, and there should a folder in there by User presets.
Please see and let me know if you see any such folder
I can get as far as appdata, I don't have that folder.
In Appdata , open the Folder Roaming, then Adobe, in the Adobe folder, there should be a folder by the name Lightroom.
I'm facing the same problem about loosing export presets (all gone), but with the OS X Yosemite in my case. I'm very disappointed with this situation since I spent a lot of time to create those export presets. Who the hell will pay for it?