Just to be sure: You let someone else put an alarm in.You left your car there, before ABS was working as it should, afterwards it doesn't?

Then there are three possible solutions.

1. Alarm wiring affects ABS so it doesn't work properly. Or the wiring causes just the lamp to indicate.
2. While they put the alarm in and did the wiring they caused a short circuit and burned down something of the ABS. Maybe just the fuse
3. While the car was there they did something else that had some kind of an effect to the ABS.

Are all ABS parts still in position?
Are the brakes working after all?