I change my display settings from 60hz to 120hz now I can't use monitor how do I reset it

I have aolder mac pro I booted it this morning and the screen was all enlarge so I went to display setting and on a wim changes the hz from 60 or something to 120hz now my screen days it does not support setting and I can't see my desktop, can I change it back without being able to see what I'm doing pls help I need to get back on ASAP