If you clean your filter pad in tap water the chemicals which kill the bacteria to make it safe to drink, kill the bacteria which process waste in your water.
So if you did the 100% water change and nothing else, it could be a difference in the pH or something else. But if you changed all the water AND cleaned the media or replaced the filter pad, you lost your BB and will be in a pretty much new cycle. There is residual bb on tank glass and a little on rocks and decorations, but not really enough to process food and fish waste.
Generally you do a deep clean of substrate say one time, the another time clean all inside glass, then another time clean the filter with treated water, either a combination of removed tank water slosh and swish or for a little better rinse, then rinse in some fresh, clean treated tank water.
That way all areas of BB wouldn't be lost at the same time. Just kind of a triple safety net.
I might also skip a days feeding and be careful to not over feed for a bit to see if they perk up.
Chances are a pwc will see them perk up. due to the water quality.