Certified GKH Gardening Expert
It sounds like your tulip bulbs need to be divided. Twenty years is long time, and all the while the bulbs have been multiplying underground, crowding each other. When the bulbs become too crowded, they usually stop flowering. This usually takes only a couple of years.
The good news is that you can easily divide them. The better news is that you're probably going to have a bumper crop of bulbs that you can spread around your garden and gift to friends. By next spring they should be flowering well again.
This article should cover everything you need to know about dividing tulips: https://www.gardeningknowhow.com/ornamental/bulbs/tulips/dividing-tulip-bulbs.htm