Go to: \Riot Games\League of Legends\game\DATA\Characters\
There 2 things can happen: A) You only have folders for the characters which you downloaded custom skins. B) You have folders for every character.
If its A) Then simply delete the folder and everything will be back to normal If its B) You need to replace the .dds for the original .dds of said character, you can find the original skin in a compressed file called "heropak_client.zip" which is somewhere in the \game or \DATA folders.
Oh, didn't noticed to messed with the particles too >_>; You can also change those in "\Riot Games\League of Legends\game\DATA" just find the particles folder and follow the same steps like in the other post, either A) or B).