If the one ovary is still viable regardless of the tubal removal or blockage, in vitro fertilization is an option and will most likely result in a successful pregnancy. This does depend on the reasons for the removal of the ovary and any other fertility problems. In vitro fertilization is about as expensive as reversing a tubal ligation.
As long as you have an ovary and tube on the same side you will be able to achieve a pregnancy. You do not need two ovariers to ovulate. It may take you a tad longer to become pregnant.
Usually in this situation you will not get pregnant but if you do then it was just Gods will for you to concieve and have a child. Doctors do not always know everything.