Two of my favorite tropicals, but I have no idea why these two plants alone in your greenhouse may have died so suddenly. Are you sure you didn't recently do something different, like fertilize them and perhaps overdo it? Thunbergia mysorensis is only marginally hardy here, and a friend in Oakland had a beautiful specimen outdoors which also died this past particularly wet winter, and maybe all the rain in March and April along with generally cool temps, when normally it would be warming up, might have done it in. Medinella is also only marginally hardy outdoors here, and I have grown M. teysmanii outdoors here only to lose it to frost one year, and this is probably not the species you were growing, as the blooms aren't as showy as the more commonly grown species.

I'd suggest trying both again...

Bahia, I know that my greenhouse wasn't too cool, I kept the heat around 60 degrees, my gas bill was $300 dollars a month. It's possible that I could have over fertilized, but I mix my fertilizer in a 55 gallon plastic drum and I watered every plant from the same mixture. These were the only two plants that just rolled over and played dead.

As long as I am not fighting some dreaded plant disease or fungus, then I will give them another try.

I have had very mixed results from medinilla, - from success to total failure. I cannot offer a suggestion as to why two different species declined so suddenly, it suggests proximity of something. If not above the ground, maybe some pest or disease attacking the roots? My area is sub-tropical so things are outdoors all year and get nothing like the TLC that I'm sure yours do, but here, I am sometimes amazed where some pests can come from - like really mean-looking scale infestasions on two vines, right at ground level.
Medinilla - in my experience - can just "go" in a snap of fingers and without any real reason. They like a free-draining mix, some species are regarded as epiphitic. I have never had much luck with them in plastic pots, I find terra-cotta much kinder in dealing with temp changes. And watering. And that means no over-kill with fertilizing. I have one in a tub - maybe 13 years? just topped up with soil occasionally, it's a bit woody but fine. Another came to me recently, it's an orange form, looked great, but the person who passed it on to me potted it on (he thought, helping) in his heavy volcanic clay soil - and probably some worms came for the trip. It ceased flowering, lost every leaf & I think is currently on its way to the compost bin.

I have found Medinilla to be extremely sensitive to fertilizers. I nearly killed one of mine recently with an application of rose fertilizer (they are growing in the ground)but was able to save it by repeated flooding with the hose to wash the fertilier away and trimming off the defoliated black stems. I think they are also susceptible to spider mites especially if you are growing in a green house with lower air movement. Perhaps you missed these as they are so small and on the under sides of the leaves. I don't know about your Thunbergia- I have always found them quite easy to grow.
I would be interested to hear what anyone else uses to fertilize their Medinilla ?
Try them again, they are both beautiful.

I would love to try growing a Medinilla...can anyone suggest a source in the San Francisco Bay Area?