If your rewards disappeared, below are some potential reasons:
Rewards are kept only for 7 days after which uncollected rewards disappear
The Clan where you played Clan Games no longer exists (everyone left)
You didn't complete at least one challenge
It is also worth noting that Gold, Elixir, Dark Elixir are added to your Treasury, and Magic Items to your Town Hall. You need to have space in the Treasury to store the resource rewards, and room in the Town Hall for Magic Item rewards. Only Gems are instantly added to your Gem count.