Take a good look around your life, and see what has happened in your life that may have been dramatic or life changing right before she started the thumb sucking. New House, new job, different schedual, new nanny, new pet, old pet died...etc.
If you can change anything back to the way it was try.
If nothing happened, then try and get her ready for her sleepy time by easing into a healthy alternative routine.
The very best thing that you can do for her is to develop a bedtime routine. Be sure to always bathe her at night almost right after dinner. Then take her to her bed if you can lay down on it, or your bed if it's too little and read to her.
Allow her to pick a book, and you can pick a book too (this is a great time to get in some real classics, especially if you cultivate this habit until she's about 9 or 10) snuggle in together with your arm around her and read the books. Afterward, take her to her room, and start a routine of saying a goodnight to her favorite stuffed animals, and saying a prayer over her. Say the same prayer every night and teach her to pray for others as you include them in your prayer. Use your daily life as things for God to bless, and this will giver her great comfort, and trust that everything will be alright.
This will give her such a sense of security that if you do this every night you will be amazed how well she will do when you are not able to be there. She will lose any separation anxiety, and by being able to count on this routine will give her the soothing she is looking for.
I know that most may think she is too old to be needing to fullfill the sucking sensation but if the cuddling and routine doesn't stop it shortly go get a pacifier (orthodontic) and offer her that when she is looking for that comfort. Give it to her when it's bedtime or nap time. Don't allow her to talk with it in her mouth or go outside with it. But you can take away a pacifier you can't take away a thumb.
Everyone is different, but I think that in looking into the way that different societies raise their children, I believe we take away the breast way too early for our children. Because really it's not just about the sucking, it is just impossible to breastfeed a child without cuddling it. So since she is obviously past the bottle and the breast she is certainly not past the cuddling, and through her actions not past the sucking either.
It is far better to allow a child to learn to soothe by using the sucking reflex than have the lost the opportunity when they are young and have the need manifest itself into a more undesireable habit when they are older.
Children who don't get enough comfort often exhibit signs of this later on by overeating, nail biting, rocking and acting out in all sorts of ways that bring negative attention to them instead of positive.
So double up on the affection and physical contact. Keep up with discipline, don't spoil her but give her lots of love affection boundaries with predictable routines.
The 30 minutes you spend every evening will bring you and your child years of happiness and closeness that no amount of money will ever be able to buy.
Best of Luck and Happiness,
S.
PS, As I read the other responses to your question, it is quite clear that the need still exists even though society says it shouldn't. My youngest still used his pacifier at night until he was in kindergarten. It's OK. Now he is the most self assured, self reliant, low maintenance kids we know.
And other mother tell me this. Give her a pacifier!