It would have been either a milk fever case, or nerve damage.
Both occur around calving time. Milk fever usually happens after calving, when the demand for calcium to go into the milk is so high, and the cow's body can't keep up. There is then a calcium deficiency. Calcium is required for muscles to work properly; with not enough calcium, the cow is effectively paralysed and can't get up.
Sometimes during calving, the calf presses on the nerves supplying the hind legs and damages them. This can be very slight, where the cow wobbles a bit then gets better, or it can be severe, with the cow unable to get up at all for several days until the nerve heals. In very severe cases the cow is put down because the nerves don't heal.
All well and good, but cows cannot lie down on the same side for more than 6 hours without nerve damage occuring. So cows with milk fever and leg paralysis have to be turned or rolled every so often so this damage doesn't happen. Better still, the farmer puts the cow in a sling and holds it upright - cow happy, legs not permanently damaged, cow doesn't have to be turned every 4-6 hours, farmer happy.