Employees who were at the restaurant at 195 Route 6 on Sunday doing cleanup and said they hoped to be open very soon.
A sign outside the restaurant said they had no water or power and would be closed for a few days.
The fire, reported at 2:45 p.m., reportedly started in the walls of the diner near the kitchen area. It took almost two hours to contain.
The Mahopac Volunteer Fire Department received support from units of the Mahopac Falls, Somers, Mohegan Lake, Yorktown and Carmel fire departments.
Nobody was injured in the fire, which remains under investigation.
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