If you're saying it's gone hard, there is a solution. Silicone oil. Get some of the purest stuff you can find(colorless & odorless) and if you have the tub version of the TIM, add a few drops in and mix it up with a stirring utensil of some sort. Should bring it back to a state that will be usable and without dramatically changing the chemistry. I have personally done this with Mineral oil(yes I know, wrong kind of oil, didn't know any better at the time) and it worked like a charm for about a year.