I've been printing all day on my little 24" Z3200 and right as I was finishing - 2 prints to go, the front panel tells me I need to load a new roll. There is a 13 x 19 sheet fully loaded.
I turned it off from the front panel, switched off the toggle switch, unplugged and went away. Came back a minute later, plugged back in, switched toggle on, the printer ejected the paper. I reloaded the paper and all was fine.
Load a sheet. Do what I did above. When it comes back on, try loading a roll again.
If it fails, turn the printer off and follow the wire lead of the media sensor switch and unplug it. Then replug the switch. Turn printer back on, then retry. If you can't get it, call a tech. You're killing youself and loosing valuable work time. Let someone qualified fix it.....