what do i lose when i am temp banned

All ranked awards, banners, hextech, and skins are stripped from the account.

For a 14 day ban you may lose Ranked rewards for this season depending on the rules they put out (TBA), though past ones won't be stripped unless it was an MMR Boosting ban. You'll lose eligibility for Hextech loot until you can show you have improved your behavior. You do not lose any skins or loot you already have, you don't lose any LP, IP, RP etc.