Yesterday I replaced an old light fixture in my bathroom, and now the new light powers on with the breaker, but the wall switch does nothing. I'm not sure if the power is going to the switch first or the light box, but when I pulled the old light off, there were three wire bundles, all black. I made sure to leave the bundled wires together and used a voltage tester to figure out which were hot. One wire bundle was hot and one was cold, and the 3rd singular wire was also hot. So I put the hot wires together and paired them with the black wires on the new light and took the one cold bundle and paired it to the white wires on the new light. I'm confused as to which wire controls the switch and where it should go.
You inadvertently bypassed the switch loop that controls that light fixture. There will most likely be a cable with a white wire that is not connected to any other white wire -- that cable is the switch loop in all but the newest installations.