I checked to be sure it was adequately filled with fluid, but that was not the problem. A few months before this problem, there were bubbles coming up through the vents at the filler plug. At that time I raised the lift arm with the handle, released the jack's valve, and let the arm fall slowly several times to purge air from the system. Finally, one day, the lift arm would not rise at all. If filling with fluid and purging air from the system do not restore function, there is probably internal leakage, even though fluid is not leaking from the jack. From what I have read, forty years of service is a reasonable time for a hydraulic jack to operate before it needs a rebuild.
A word of caution: Rebuilding this jack presented several challenges that seemed almost insurmountable at the occurrence of each. Rebuilding this jack was much more difficult than simply replacing a few "O" rings, and it required more than the very few hours some say are required for rebuilding a jack. Further, I had to make several special tools to get the job done. If you want to attempt rebuilding a jack and you are not a member of Instructables, I would encourage you to pick a password and a screenname, and join. Doing that will allow you to download a PDF of this Instructable for printing, or to view at any time later on your computer, assuming you wish to consult what I have done as a guide.
When putting fluid into a jack never use anything other than hydraulic jack fluid. Do not use motor oil or brake fluid. Brake fluid makes the seals swell.
At this link you will find one man's description of how he rejuvenated his twenty-five year old jack by flushing its insides with a solvent, letting it dry completely, and filling it with fresh hydraulic jack fluid. It could be worth a try. I did find one manual for a floor jack that said the fluid should be changed every year. The procedure is to place the jack over a large pan, remove the filler plug, turn the jack on its side and let it drain. Then fill it again.