On a rollercoaster, one of the cars stopped. So basically they're just sitting there in the carts, and none of the other cars will come back. It's losing me money >:T

Go to the ride window and double click the red stop light. That should reset the ride and force the guests to exit the ride. It sounds like there is something wrong with the ride (missing track or something) so whilst the ride is turned off, check it, edit it, then test it. If it then seems like it is working again then turn it back on with the green light.

Alternatively, has the ride just broken down? If it has, do you have any maintenance men to fix it?

I had this with one of my custom built coasters. Essentially, I had removed the minimum wait time for a car to leave which was causing it to run into the back of the car in front of it during the ride, causing it to stop. Once you take the steps that Korell mentioned, you might be able to prevent it from happening again by increasing the minimum wait time for a car to leave.