I received a gift card for $50 from a friend. When I tried to use it the restaurant said it had nothing on it. Please help me resolve this.

Get the receipt showing the card was activated.

The card likely wasn't activated properly when purchased, or may already have been redeemed prior to your attempt at using it. The merchant is usually able to tell if the card wasn't activated at all, or was activated and redeemed. This can be done in the store at a register.