Network Place disappeared

I have 3PCs and a Laptop running on a "Home" network. For years I've had no problem. Let's call them 1, 2, 2a and L. 2a was the nominal host but it doesn't seem to matter. Suddenly the other 3 can't talk to 1 and 1 can't talk to the others. They are on a BT Hub3 router. 1 gets its internet with no problem. I have tried re-setting up a network to no avail. If I try and access 1 from the list of computers on the access it says that there is no permission. Simply going to "My Network Places" from 2, 2a or L shows the other 2 but no 1. Trying it from 1 shows nothing.

I'm stumped. Help gratefully received.