It is not normal to power servos from the USB connector of the computer. Depending on the servo's size this can pull too much current.
However it sounds like your wiring has gone wrong. Are you using solderless bread board? This is not a very good method of construction and wiring that looks like it is making contact often isn't.
So get your multimeter out and measure the voltage across the servo's supply at the servo motor itself.
The other thing to try is to use an other pin to control the servo as you might have damaged pin 6.