The salad may not have been frozen intentionally, it may have been placed too close to the cooling unit in a fridge, or placed in a malfunctioning fridge. You should go after work and mention it to the manager so he can take appropriate corrective measures.

And if it was frozen intentionally, let him know that - uh-DUH! - that will ruin a salad.

Keep the reciept. He may offer to give you a refund even if you don't take the salad with you.