I was going to make some cake balls yesterday so I took out some frozen pieces of cake and crumbs from the freezer. I realised I didn't have enough time and there was no way I can make them that day so I put it back in the freezer about an hour - 2 hours later (by that time it had defrosted). Is it safe to use again? I was going to make it for family and friends

Check for freezer burn, that could mess with the taste, but yes, its safe either way.

What is freezer burn? Will I know it when I see it?

Frost/ice crystals on the food.

Is there a way of preventing that?

Just wrap it well with saran wrap and freeze in an airtight container.