Unhide empty measures

Same way you hid them to begin with. For example, if you used Format / Style / Hide empty measures, just go abck there and turn the option off. Or if you used Staff Properties / Hide when empty / Always, same. If the part was completely empty so there is literally nothing to right-click to bring up the dialog, that might seem tricky, but you can use the arrow buttons at the bottom of the staff properties dialog to move through all staves. So just bring the dialog up on any visible staff and then start arrowing.