@Karen3524 If it was a failure of your plumbing and NOT the fault of the guest breaking something through misuse, a refund is certainly due.
If it is the only bathing source in the property, then a full refund for that last night. If there is another bathroom, or the bathtub plumbing is operational, then 50% of the affected night.
If it was the fault of the guest and the plumbing is not functioning, the guest needs to agree to a reasonable cost for repair and must tender that amount immediately. My plumber charges $150 to just show up to the property to evaluate the problem, so I would want at least that amount from the guest.