How can I fix an office chair that is now too low and won't "pump" up higher? - high office chairs

I go to nursing school (at age 56, of all things!) And an office chair that I paid so much for the year 2000. Yes, I know that she is old and needs replacing, but do not afford this. The height should be adjusted, but it is now going so far and is not higher like. It killling my shoulders to do homework on my desk, because now the table is very high because my chair is too low. I hope this makes sense? Is there a way to correct this chair, it will be higher? I even thought of a platform to increase the president of all, but simply not feasible. Has anyone a cure, right? And I thank you for your help!