If the amp was on and the speaker cables not tight you created an ever load of resistance to the circuit ,check the fuses first ,sometimes you get lucky see outside fuses, and inside with a volt meter pull each one out while unit s Unplugged .
We all make mistakes ,Always make sure all cables are secured good before turning the unit on Even swapping a digital usb cable I turn everything off first.
It doesnot take much to cause a spike or surge , that is most likely what happene.