USB stopped working

Hi.
I bought this 3D printer used , and the previous owner did several up grades to it including a 24 volt hot plate .
I think in setting it up I shorted out something in the 24 volt system . I decided to change everything back to the
15 volt DC PSU . Everything seams OK , but the USB refuses to talk to the computer . The computer doesn’t even see it.
It did before this happened . Am I going to need a new cpu board ?