I assume that either the Mac or the PC (or both) have been able to connect with the UNO in the past.
Plugging in a 5V supply to the USB will not normally damage anything, but if there was a problem with the supply or some sort of transient occurred when plugging or unplugging then it is possible that something got fried.
Was there enough code already loaded in the UNO to demonstrate that the processor is still running when you power it from USB (even though you can’t load new code)? If so, then it is possible that only the bootloader was damaged. You can install a new bootloader using ICSP. See:
Installing an Arduino Bootloader - SparkFun Learn
However, that needs additional hardware, and if you don’t already have something suitable then it would be easier (and probably cheaper) to buy another UNO. Then, when the assignment is submitted and you have the time, you can use the new UNO as your ICSP programmer for the bootloader and possibly bring that faulty device back to life.
