More information may be needed.
Turn off the breaker to turn off power to load.
If this is a mechanical timer, and the override lever is not working, turn off power at the breaker.
Use WD40 to see if you can loosen the lever.
With mechanical timer, usually when override is not working, it signals you need new timer.
Add a comment and describe timer and what you have tried so far and what solved the problem.
Add a comment with in-focus photo of your timer posted on Flickr for possible identification and more complete help.
I cannot find reference to York timers on the web to recommend a replacement timer.
If timer is 120V then Intermatic T101, or if timer is 240V then Intermatic T104
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.htmlIntermatic competitive reference crossover guide does not show York brand timers as of right now.
You might have a Tork timer?
http://waterheatertimer.org/Tork-timers-and-manuals.htmlUpvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.