Is anything connected to reset, pin 0, or pin 1 of the Arduino? Those pins must be clean during the upload process
If you briefly ground reset (the pro mini's don't have a reset button do they? If they do just use that), does the LED blink? If not, the bootloader is gone, reburn bootloader using an ISP programmer.
Does the LED blink just as if you'd grounded reset during the upload process? If not, suspect a problem with the DTR reset circuit.
Do you have the right processor selected? Wrong board/processor will generate similar errors.