Older software for the Edge 500 can cause the device to either lock-up or get stuck in a constant power cycle, resulting in the device to never progress past the startup screen. The cause is an unreadable file placed in the NewFiles folder.
The device must be held in mass storage mode to access and delete the NewFiles folder:
The device will now proceed past the startup screen. You will need to update the operating software.
Update - http://connect.garmin.com/firmware