What camera were you using? There's now an option in the Crop tool to reset the crop back to the full camera resolution, but that's for newer cameras. IIRC, that option is not available for earlier cameras such as the Canon 5D MkIII.
Which is what I thought might be the case. You could try the DNG Recover Edge official Adobe plug-in, which is what was available before they provided the current functionality for later cameras. It's still listed on the Adobe web-site, so should in theory still work: https://helpx.adobe.com/uk/lightroom-classic/plug-ins.html