I had the same thing happen to me last weekend.
Pushed off to attack a 2nd serve and felt as a tennis ball had hit the back of my calf from behind. Felt as if part of the calf muscle had snapped width ways like an elastic band. Went down in a crumpled and the calf muscle completely seized up. Couldn't put any weight on it at all or move my ankle (as that would use the calf muscle).
Saw a doctor the next day, and a physio the day after. Calf muscle tear, possibly rupture. Prescribed crutches, rest, ice, elevation, anti-inflamms, and some stretching / strenghting exercises of increasing intensity as the pain reduces.
5 days after the injury I can walk without crutches which is a good sign, though the walking has restricted movement. Still have to be careful going up and down stairs or putting too much pressure through the calf muscle.
Physio thinks I could be walking again normally again within 4 weeks, and back on the tennis court fully fit within 6-8 weeks.
Useful info in this thread. I think I'll be cautious in coming back, and not rush it.