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Answer

There are a few reasons your tomatoes may not be ripening including temperature, nutrients (fertilizer), sunlight and overcrowding are the most common.

Temperature - When daily temperatures are averaging 86 degrees or higher tomatoes will stop producing the chemical compounds that makes them ripen. Tomatoes turn red and ripen best around 70 degrees.
Nutrients - It takes a significant amount of energy for fruit to turn ripe, regular fertilizing of your tomato plants will ensure they are receiving the proper nutrients. I like using an organic tomato specific blend that can be worked into the soil surface and watered in the soil. However any balanced formula of N-P-K (Nitrogen, Phosphors, Potassium) you like can be used at the recommended rate.
Sunlight - Tomatoes should be planted in a location in the garden where they receive between 7-8 hours of sunlight each day, less then that can slow the development process.
Overcrowding - Too many plants in the garden can cause competition for resources; nutrients, light, water.

If these do not sound like the cause try removing a portion of the unripe tomatoes, this will free up some energy for the remaining fruit. The crop maybe smaller but perhaps they will begin to turn red, ripe and delicious. Remove the fruit that appears most ripe you maybe able to ripen them in a paper bag with a banana or two, bananas produce ethylene which helps ripen other fruit.

Remember most tomato varieties require at least 65-75 days of sun before they will begin to ripen.

Happy Gardening!

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