OK so something happened and it is just coincidence it happened now, but was going to bite you at some point anyway and we get a post about it almost every week, is that the IP probably changed on one of the Amcrest.
One of the updates BI made that was trying to make life easy for those that don't understand IP addresses and was using DHCP and was tying the camera to the MAC address instead and then would potentially give you a different IP address that BI didn't see.
In theory it should be a good feature because many people use DHCP. Most consumer cams are dhcp by default.
So
blueiris is able to follow the camera if the dhcp lease get revoked, but it can also cause problems.
Make your camera a static IP address and then check the Skip Initial MAC, HTTP, DNS reachability tests
Here are a few threads that discuss it.
Blue Iris is changing IP addresses randomly
BI changed camera IP addresses
New BI doesn't return the signal for some cameras after router/switch reboot, gets stuck at no signal, was fine on old BI