Can you be more specific about the type of photos and the steps you are taking to achieve 'merging'? (screen-clips?)
Pano- Do the images overlap? Is it a horizontal or vertical pano?
HDR- Are the photos framed identical? Were the different exposures set in camera?
Lr Version Number?
They r just 5 ABE raw files shot with the DJI Inspire. All 5 photos were taken at same time. I have done this before without issue. But this time I can't, nor can I merge 4 photos taken with 50% overlap horizontally they will not merge either. It doesn't even try. As soon as I select merge LR just freezes.
I recently had a Pano totally freeze (like you, CC2015.8). It was a 'Vertical' pano with the camera held 'landscape' and rotated upwards on a waterfall. Lr would not create the pano from the 'landscape' frames. I rotated the photos 90degrees in Lr so they appeared as vertical frames- then the Pano merge worked! (Rotated the final pano back 90degrees to appear upright)
You say "I have done this before..." so I cannot say what the answer is, or what may have changed.
Re-setting the Preferences file (by a rename or delete) is often suggested in such cases. Resetting Preferences in Lightroom CC/6
Also I think Lr likes the photos un-touched by edits in the develop module, ie. try merging the images 'Reset' as at import.
Have you tried exporting each file to JPG and attempting the merge with these.