Yeah, if the stem is clean broken or the stem tissues are obviously ruptured all the way across, you are better off cutting it and trying to root it, though if it is still attached by at least 1/2 of the stem diameter, the plant can be repaired -- it's like setting a bone... Align, splint, and tie.
Mine will develop a bump after the plant re-constructs the kinked tissue where it is obviously bruised with internal damage.
It's important on discovery of bent over stem NOT to keep moving it about by trying to lift the heavy top up
and failing -- I lost hold of mine once when I first straightened it and leaned it on the bamboo stake to get the string that I hadn't cut yet
***cut your string and have it ready first***
Here is a reason to save at least one blooming/fruiting side shoot to take over. If there is *nothing* below the break, assuming there are sucker nodes to grow, it will take about two months for the first ripe fruit (two weeks to grow a shoot and bloom/set fruit, then 6 weeks to ripe fruit). So you have to look to the end of the growing season and decide if there is enough time.
Better to support your tomato plants well BEFORE an accident like this.
Once the first little green fruits start to show, consider it top priority -- those fruits will weigh the stems and branches down... and remember that the plants can get a LOT BIGGER.
Before heavy rains (water on the foliage can weigh down the stems) and gusty wind storms, be sure to inspect and tie up any unsupported new/long shoots. In my case, I think Casey collapsed after a heavy rains caused build up of internal pressure, then the pressure dropped when the weather cleared up yesterday. I had been lulled into thinking it will be OK because it was holding up to the wet foliage and wind.