We had a plumbing problem and water leaked into the garage through a light fixture. Is this a fire hazard?

Let's begin with a certainty...electricity by itself is a fire hazard.
so fix the plumbing, replace the fixture and hire a professional to do the work...

Water and electricity are not a fire hazard. Water and electricity is a corrosion/rust hazard. Things like circuit breakers will get corrosion inside and not work properly quite quickly. Things like light fixtures will corrode rust over time and will fail.