It could have been as simple as someone redid their roof and closed up a hole the bats were in or a tree got cut down that housed them.

If you want to help and you are in a position to put a bat-house up on your home or plant native plant species that attract bugs for the bats to eat. Some bat species like moths and beetles. It really depends on where you are in the world to what you can do to help, look for a local bat society or wetland group and ask what you can do.