How to avoid losing your item sets when repairing your client
The problem is that Item Sets are stored on your computer rather then the Riot servers. So when you repair your install it reverts all files back to their original state. The fix for this is simply to back up the Item Sets before you repair. To do this you must go to:
C:\RiotGames\LeagueofLegends\RADS\projects\lol_air_client\releases\0.0.1.16\deploy\preferences
((Note this is the default installation, if you have a custom location then you must navigate to that))
Once there you must back up the .properties file of the account(s) you want the Item Sets saved on. Then once you have repaired your client place the .properties file back into the repaired folder.
This won’t help if you have already lost your Item Sets, sadly this means you just need to set them up again from scratch. We can also look towards an Item Set wipe, if Riot ever decides to move them to the Riot Servers like they did with Masteries.
Credits:OceloteWorld.