Are you using the mini USB port or the I/O port that looks like a headphone jack (yours may not have a I/O port)?
Just to double check, you did replace the coin cell backup battery as well right?
Check that there's no debris in either port. Also check for any damage inside the ports like mangled pins.
If you are installing the OS from the USB port, hold [Stat] while removing and re-inserting a battery. This will force the calculator to ignore the I/O port. It will enter recovery mode.
If that doesn't work, try getting a SilverLink cable to use instead. Hold [del] while re-inserting a battery. This will enter recovery mode too.