5 Answers

Some advice here>>http://www.bhg.com/advice/gardening/vegetable/why-are-my-tomatoes-not-ripening/

And within the bag, you can include an apple, the ethylene from the apple will hasten the ripening of other fruits and veggies.  The exception is if they are not even close to maturity, then they will not ripen, but die. 

Ducky

:(

Lack of sunshine and warmth would be the first reason, may be planted too late to mature in time, before the summer ends.

Pull em off and put em in a brown paper bag. They will ripen there.  Unless of course as Nomdeplume says, depends on the variety.  Put yellow ones in a bag they turn brown.

Not all tomatoes turn red when ripe, some go yellow and some are green when ripe, it all depends on the variety ; Golden sunrise is a very good yellow one and green grape is the sweetest of all.