My brake got stuck?

[David]

On Wednesday morning I started the Boxster solely for the purpose of washing the car. All I did was back it up to a shaded spot on the driveway, and gave it a good wash. As soon as I dryed it I pulled it back into the garage, where it sat unused until this evening (4 days later). I started it, released the parking brake, shifted into reverse and tried to back up. There was resistance, and a horrible noise. I stopped, took it out of gear, then back into reverse and tried again. Once again, resistance and a horrible noise. This time I kept going and, after a loud "thunk" everything seemed to free up. I drove for about 30 minutes or so and everything seems to be just fine. Any idea what this could be and should I worry about it?

Many thanks for the help!