On most networks, the internet router is also the DHCP server, meaning it provides IP addresses to any devices that aren't configured with a static IP address.  If a new router is installed, it may have a default IP address that is on a different subnet from the previous router.

For example:

Old Router IP address: 192.168.1.1

New Router Default IP address: 192.168.0.1

Because the subnet of the old router is 192.168.1.x, all of the DHCP devices will have an IP address on the subnet 192.168.1.x.  But when the new router is installed on the network, all of those DHCP devices will now have an IP address on the subnet 192.168.0.x.  If all of the devices on the network were using DHCP then this wouldn't be a problem.  However, there will be some devices on the network which have been assigned static IP addresses on the subnet 192.168.1.x.  This creates the issue because devices on different subnets cannot communicate with each other.  While it may be tempting to change all of the static IP references to match the new router subnet, DO NOT DO THIS!  This is a very time consuming undertaking and there is a much simpler solution.  The best approach is to access the new router in a browser and reconfigure it to use the same subnet as the old router.  This will typically involve changing the router's IP address to match the IP address of the old router but could also be configured in the DHCP range.