More details would be helpful, such as brand and style. Are we talking a key-in-knob style? If so, since the plug's already rotated, see if you can pop the lever off and remove the cylinder. Should be an easy fix from there.
If we're talking commercial-style mortise, there are a lot more variables. I've seen situations with an entry-style wherein 1) the deadbolt is thrown, 2) the plug is turned but not to the degree where it's interacting with the turn hub, and 3) the interior (single-motion exit) lever is operated that allow the turn hub and cylinder tailpiece to get out of sequence, trapping the plug in a rotated position. At that point, you're looking at removing the mortise cylinder to correct.
All the above presumes you're reasonably familiar with locks and hardware. If there's a question, call a locksmith. Cheaper in the long run, because believe me, you could do more damage from here. I'll leave the admonition a about not picking a lock in use to others; all I'll say is hopefully this is a lock you own

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