The GARDENA gardening expert
It is difficult to say sight unseen, Mrs. K. In essence, there are two reasons that need to be checked:
Did anything unusual happen to the plants, such as overfertilization, urine or droppings from animals? Are there any traces of feeding on the base of the stem or roots? Then the culprit can be easily found and action could be taken against it.
The other reason, and possibly in the right direction, is a soil-borne fungus or more specific, an infestation of the wilting fungus. This can be brought in with young plants but also through overwatering of the soil and originating in your own greenhouse. If the other tomatoes die, this would suggest a fungus infection. In this case, I would advise a change in cultivation with other vegetables (no solanaceae plants such as tomatoes, aubergines or Cape Gooseberries). If the problem persists, you will need to consider a soil infection or a changing out of the soil. Before you begin such a huge undertaking, please contact me again so that we can target the problem.