Why is my abs light showing after I change my brake pad?

You likely damaged a wheel speed sensor. The body computer (BCM) works the speedometer and probably gets its signal from the engine control computer (ECM) which probably gets the signal from the antilock brake (ABS) module or it could come from the transmission. Disabling the speedometer is likely done to encourage the owner not to ignore the ABS light. This is a common strategy with modern cars. This is quite convoluted but this is how manufactures discourage people from fixing their own cars. You could try taking the wheels off and make sure the wiring connector is still plugged into the sensor. Otherwise you need to see someone with a diagnostic scan tool to determine why the light is on. Fix the ABS first to see if that restores the speedometer function.