Resources cannot disappear as we have their information stored on a separate server.
However, if other users defeat you in Monster Arena they can steal resources from your account.
Using a shield can protect you from enemies.

There are two types of Shields and they are explained below:

A Shield makes you invisible to Random search.
You can pay a certain amount of gems to buy Shield protection for 1, 3 or 7 days.
Automatic Shields:
If you are attacked and you lose, you will automatically get a 16 hour shield.
If you attack someone whilst having an active Shield on your account, you will lose the Shield and will be subject to attacks.