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Make sure you don't have another common issue first... Blossom end rot... It is a magnesium/calcium deficiency. Dolomitic lime (agri-pel is what I use) will take care of the issue if it is that... Blight to a tomato will mean reconditioning your soil, and ridding it of the issue. wettable sulfur will help there.

I'm guessing it just rained really hard? Usually it is caused by a change in moisture/humidity. Sometimes it even happens after it gets really dry and you water it really good. Make sure to maintain proper soil moisture. Even though tomatoes are drought tolerant, doesn't mean they like it. Also, add dolomitic lime (agri-pel is the brand I use) and it will save you a lot of trouble in the future. Try adding a liquid silica to the water as well. Just be stingy as this will hinder the uptake of potassium, which your fruit need to bear properly. If you have any more questions feel free to shoot me a line at 6.schuster@gmail.com or just ask here, and I'll be here to answer. Thank You!

There are several tomato diseases which could cause the symptoms you describe, but fusarium wilt and tomato black root rot seem like top possibilities. Fusarium is a fungus that damages the roots and stem and causes yellowing of the leaves:
https://www.gardeningknowhow.com/plant-problems/disease/controlling-fusarium-wilt.htm

Black root rot caused by Thielaviopsis fungi is less common but has been seen in California. Check this fact sheet for more information:
plantclinic.cornell.edu/factsheets/blackrootrot.pdf

And here are a few more tomato diseases in case those don't match:
https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-diseases.htm

In any case it is best to avoid growing tomatoes, potatoes, eggplant, and other relatives in the Solanaceae family in that pot for the next few years. You may want to get rid of the soil in the planter entirely just to be safe and then use 10% bleach to clean the planter.

You can add a high phosphorous and potassium fertilizer, and that will help further ripening. This article will help:https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-fertilizer.htm

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We have many articles to help you with Tomatoes and the related diseases.

https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-plant-problems.htm
https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-diseases.htm

Please feel free to take a picture of the tomato plant, describe the issue you are having and send to us! We would be happy to help you!

There is a recipe that I like to use for both preventative, and spot treatments. It is one of the best I have found. You don't have to have all ingredients, but the more you have, the better it works
:First, you will want to prime the ground with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.