Hi there!
When a USB cable is plugged into your board and computer, the computer powers the board. When you turned on the power to the printer, it also tried to power the board, which most likely fried the communication chip. Some genuine models have protection circuits to prevent this, but clones do not always have this in order to reduce cost.
Since you said that when the board is powered the LED is blinking, the ATMega2560 chip could still be functional, but the converter chip is probably dead. If you can replace the chip, great, however you would have to find a chip that already has the proper firmware for operation with the Arduino IDE. Normally you could preload the firmware using the ICSP header, but you said your board does not have that for the communication chip.
That being said, it may not be worth the effort to try and repair the board. You can upload sketches to the main chip using its ICSP header (if it has one) and use the USB port just for powering, but the serial communication may not be possible anymore.
Good luck!