Question:  I live in South Florida and I started growing tomatoes in January (inside in containers).  I moved them outside in April.  In May, the plants looked very healthy and tomatoes appeared.  Not a single tomato turned red and the plants look like they are dying.  I did add fertilizer in April and May and I water them 2 times a day (early morning and early evening).  What did I do wrong? 
Answer:  Well, you give me little information other than the plants look like they are dying, but I'll take a stab at it.  You say you moved the plants outside in April.  If you left them in the containers and are watering twice a day, it could be that you are giving them too much water, especially if the containers do not have drainage holes (or maybe even if they do).  If you put them in the ground, in South Florida two times a day might be reasonable, especially if they are in extremely sandy soil.  Bear in mind, though, that established tomato plants have fairly extensive root systems and can tolerate a good bit of heat and drought, and in fact a somewhat drier soil is usually beneficial during the fruiting stage, which is where it sounds like yours started failing.  As for fertilizing in April and May, it could be that you over-fertilized.  The best bet is to buy a slow-release fertilizer specifically for tomatoes, or one for vegetables in general and then fertilize every three months or so.  Lastly, assuming you don't see holes in the leaves or obvious insect damage, it could be a tomato disease such as Verticillium Wilt, a fungal disease that is the bane of tomato growers everywhere and that slowly but surely compromises the plant's health.  If that is the case, you will have to destroy the plants and start over.  The good news for you in South Florida is that you have a very long growing season, and if these plants fail completely, you can start all over and still have a bumper crop of tomatoes by mid summer.  If you do this, make sure you use new soil and a clean pot if you are putting them in containers, or plant them in a different place if putting them in the ground.  Read the labels on the new plants and make sure that you buy resistant varieties for all the common tomato problems (look for VFNT on the label) - standing for Verticillium wilt, Fusarium wilt (funguses), root-knot nematodes, and tobacco mosaic virus.  Your local garden center will likely carry a nice variety for your area

Question:  My tomato plants are dying from the bottom up what do I need to do to save them. they were fruiting and suddenly start dying.
Answer:  Unfortunately, yours are classic symptoms of either Verticillium wilt or Fusarium wilt, both soil-borne funguses that are systemic (are carried through the vascular system of the plant).  Once a plant is infected, there is no cure and the best thing you can do is pull the plant roots and all and dispose of it in a sealed trash bag.  Removing the soil in the area where the plant was growing is a good idea too.  After that, don't plant anything that is susceptible to these wilts in that area at least until next year.  After pulling the plant, slice a stem open and if there are brown streaks within, it was Verticillium, and if there are reddish brown areas, it was Fusarium.  Good gardening practices such as crop rotation go a long way in minimizing problems such as this, as does careful plant selection (look for VFNT on the label for tomatoes and other vegetables- see post above).  I hate to be the bearer of such bad news, but do be aware that problems such as this happen to everyone regardless of how experienced they are, so don't let this setback discourage you from trying again.