This is admittedly a strange error, but it's occurred twice so I'm going to submit it. I've had two panel variants on separate pages which stopped displaying their contents without showing any kind of error. The first one I simply recreated from scratch, but when the second failed I became concerned that something was corrupting panels. In either case I didn't change the panels, the only relevant change I can think of is that I've been updating panels, views, and ctools dev releases. The second failure case (attached) is in node views, and it is only one variant which doesn't display. There are three other variants which use the same (custom) layout which display just fine.
The failure scenario appears as follows:
The title will display, and can have any substitutions (no matter how complex). But none of the content below displays. I've tried disabling all the content in each panel, and adding something out-of-the-box (like Page title), but that doesn't display either.