I took my car to a mechanic to have the brakes inspected, and I was
told I would not be charged for the inspection. When I decided not to
have the repair work done there, they said they were going to charge
me for 1 hour of labour. Also, I did not sign any contract of labour
beforehand. Now they're holding the car until I pay them for 1 hour of
labour. What legal recourse do I have? I need this answer as soon as
possible, because I can't drive until I get my car back....

If you were told you would not be charged, that's the end of it right
there. I'd notify them, that you're picking the car up later today.
When you get there, if they did not release the car, I would call the
cops, and have it mediated right then and there. I wouldn't threaten
them beforehand at all, because there's a chance they could/would
sabotague your car out of malice. So just go there, try to work things
out and explain your situation, and then call the cops. (But that's
just me, I'd wait for others' opinions)