What do you see in the Layers panel? Or does Photoshop simply not complete the merge? Are you using more files than previously, or from a new camera with a larger sensor?
What are your system specs? (How powerful is your computer?)
If you are using Windows, try opening the Task Manager and view the Performance tab to see if you are maxed out on CPU and RAM, and if it eventually drops to normal levels.
Try downsizing all the images before merging them.
Try loading the images into Photoshop layers. Select all the layers, and go Edit > Auto align layers followed by Edit > Auto merge layers with seamless tones and colours checked.