It should be very unlikely that the NIC cards all fried simultaneously, so
I'd argue the most likely alternative is a bad cable (or terminator).

Unplug the cable from every station (leave the T connectors in place), then
go to one end of the cable, remove the terminator, and use an ohmmeter to test
the resistance between the center connector and the ring.  It should be 50
ohms, and definitely not 0 or infinity (one of which I'll bet you'll find).
(If it is 50 ohms, swap terminators and measure again, maybe you removed the
bad one. :-) )  Don't unplug all the segments before checking, because you
don't want to disturb anything (a snapped wire might rejoin if you move it
just right).

If you can't find an ohmmeter, go buy a new length of coax and two new T
connectors and two terminators, and hook up two machines with entirely new 
cable and see if the problem goes away.  (Hey, everyone needs spare network
parts!  On the other hand, everyone needs an ohmmeter!)

If that fails to show an obvious problem, carefully inspect each of the BNC
connectors on the NIC cards and the BNC plug on each T connector; especially
watch out for center pins which have been pushed back into the connector, they
should be approximately flush with the top of the ring.