My tomatoes started out very healthy but a couple have died off. I was left
with 3 plants that I thought would be very healthy but now two of them are
succumbing to the same problem the others had.

At the base of the plant, the stem is going brown, thin and hard. It seems
to be choking the plant to death.

What is causing this? Can I bring them back to life? What should I do next
time to stop it happening?

Thanks in advance for your help.

Cheers
Petesin