Any known reason: e.g., factory reset, power outage, firmware update?
Are there logs in the router? May or may not be available but if so they probably would also have to have been enabled. Worth a look.
Getting back to normal will most likely be a complete "do over".
First get the router reconfigured as before. I.e., its' IP address, subnet mask, available DHCP IP address range, etc..
Then, if necessary reassign the cameras' static IP addresses one by one. Be sure that the IP addresses are outside of the available DHCP address range and reserved via each camera's MAC.
Be very methodical: router and network running, computers and other devices up and running, then cameras.
Watch for problems as you go and keep checking back...