Items don't normally disappear, unless it's from an Airdrop! The nature of the airdrop is that it’s limited - it’s limited to the first few people who claim it and it’s limited that you can only claim it within a minute of its drop. If you have not claimed it by this time, then the item would disappear.
There are also instances where you would open an airdrop crate and it would expire before giving you an item. This would be due to the timer running out right before you opened the crate. Please note that items that come from airdrop crates have to be claimed before the timer expires.
You may find more information here: https://community.sliver.tv/airdrops/
Sliver will not return expired items, but we do wish you luck in the future airdrops! Airdrops happen frequently which means you have a lot of chances to win