" The shortish answer is that you have an IP address . A unique one that your provider (say, XYZ Hosting) "loans" you, in most cases. This unique IP allows you to get on and use the Internet. That IP address is one of a block, or bunch, that belongs to your provider. So, if you're the poor tech running ABC Pie Company and you get attacked from a user. You lookup the IP address of the offender and find it's one of the "block" of IP addresses assigned to XYZ Hosting. Well, your boss want's this activity stopped, cold. So, knowing that a user could get ANY IP assigned to them from that block, you block the whole range. Brute force. Not exactly efficient, but effective. That wide net you cast catches some innocent users. It happens. VPNs are sometimes blocked, not just for bad actors, but simply for the fact that they are private -- to some degree or another -- and defeat all the goodies that companies use to track and such.
The probably a host of other reasons an IP gets blocked. It might be a temporary measure that was put in place during an emergency and will work itself into a more effective block. The hosting provider may move all the users to a unblocked chunk they own. It's ugly, it happens. If you're working closely with a business that has your business blocked, they can use a "white list" or similar to allow access -- again, brute force."
