This is just a wild guess, but it might be worth a try.
All Canon DSLRs have a sensor switch on the battery door, that disables the camera when the battery door is open. On the 50D, this switch is in a small recess on the edge of the battery compartment opposite to the door hinge, easily visible when the door is open.
The switch may be stuck in the pushed-in position, maybe due to dust or dirt. This may lead to the camera not being powered up correctly when you insert the battery.
Try prodding the switch plunger with a toothpick or similar implement to see if it's stuck, and maybe add a tiny drop of contact cleaner or isopropyl alcohol to aid in getting things moving again.