Refund off the menu

I booked a table for 12 at a restaurant to celebrate my sister’s birthday, paying £60 deposit. But we had to call off with a week to go when she took ill. The restaurant accepted the cancellation but withheld the deposit. Is there anything we can do? Check the terms and conditions of the booking. But if they rebooked the table, I’d question what their losses are. Speak to the manager about using the deposit against a future booking once your sister is on the mend.