I am trying to print an earbud holder with my Prusa i3. About an hour though the 3-hour print I left the printer for about 20 minutes and when I came back the printer stopped printing. The printer didn't display any errors on the LCD. The extruder and printing bed all cooled down but the z-axis didn't raise so the nozzle was stuck to the print. What could cause the printer to stop printing?
There's really no telling why that happened if you weren't there to observe it.
A possibility is a temporary power outage, which would stop and reset the printer without any trace of it having happened. Even if there was not a power outage, maybe there was a temporary dip in power that caused the power supply to be unable to supply the required voltage (or perhaps the power supply was of poor quality to begin with and suffered some issue that caused the voltage to drop).
Another possibility is that you were printing via a computer, and the computer rebooted during the print. If this is not the case and you were printing via an SD card, maybe the SD card became corrupted and the printer read some invalid G-code and reset itself (though this latter case would probably come with some indication of a fault on the LCD).