That is not good news
You did not say what year your C5 is, if 97-98 the repair would be easier than as starting in 1999 the ABS and EBTCM was moved to the front of the C5
near back side of radiator which then faces the front of engine where PS pump is
Never heard of the pulley of PSP come off as the pulley is pressed on
The question is what caused that to happen such as looking at the damper pulley off the crank got loose and that put a load on
the belt, which then put force on PSP pulley
IF your pulley face has an opening like this, the job will be easier as can get to the bolts behind it
If the face is like this, then it will be harder to work on
This video might help you plan as what has to be done
Trying to press that pulley back on while pump is still installed might be a real bitch and might be better going through all this is
to replace with a new PSP/pulley
Assure the belt is good or replace it,
Check all idler pulleys are good and make damn sure the damper pulley off crank is not loose (big job if it is !) or it is bent and needs to be replaced