I turn on the camera, everything works, all the buttons, dials, focus, etc. As soon as I take a shot, the camera shows the image in the screen, then everything shuts down - all the buttons, dials - focus still works, but I can't shoot. I have to take the battery out and put it back in then I can take one more shot.
The problem started yesterday while using a Sunpak DigiFlash 3000 - I assume this is related to the camera problem.
The problem persists whether I am trying to use the flash or not. All settings reset to default. Flash disabled. Still doesn't work.
Obviously the camera could simply be broken, but it does sound a bit like the battery is knackered. Lithium batteries will eventually wear out, so that even when fully charged, they don't have enough capacity left to do anything useful. Or in your case, grab just one shot. The flash may be a coinidence.
So maybe try a new battery?
Don't think it's the battery. I've tried 2 different batteries, both new, both fully charged. Same problem regardless of battery.
I still think it's related to the flash, the instructions for the flash (which I now can't find) said something about the importance of turning the flash on only after the camera is on, and turning it off before the camera is turned off. I think my camera my have shut off with the flash still on, or maybe I even did that.
If the flash is still attached, remove it. Do the issues persist? Also, try resetting the camera and try again.
As stated above, the camera may now be broken.
The flash you have is incredibly inexpensive. But to me, that's a warning sign that sub-par components are being used. I do wonder if it has damaged your camera in some way.
I have not seen any Canon specs on what the sync terminal on the hot shoe is rated for but the PC sync terminal is rated to only 250 volts on the 1 and 5 series and I suspect that applies to the hot shoe also. A failing flash that exposes the sync or communications lines to the Xeon tube firing voltage can easily cause damage to the camera.
I owned a Sunpak flash a LONG time ago with a Canon AE-1 and it was a decent and well made unit. The 3000 doesn't seem to fit that category.
Does the camera not show any error message when it locks up?
If anyone is interested in how this story ends - I know I am - here it is.
Drove 3 hours to the repair shop. Lady at the counter tries it out - it works. I try it out - it works. Driving home 3 hours I realize that I took the CF card out before taking it to the shop. After trying several combinations of card selections (CF, SD, CF & SD, RAW, JPG) it seems related to the CF card, but can't replicate the problem reliably. Now the camera works most of the time, refuses to work at others. It's an improvement.
CF card pins are far too easy to bend, especially if you fall prey to one of the very common counterfeit knockoff cards. There is very little room for error and a card slightly out of tolerance will easily result in a bent pin.
I primarily use the SD card slot in my 5DS and 5DS R bodies with the CF card only in use on the rare occasions where I want the redundancy of having the RAW files recorded to both cards. My 1DX, 1DX II, and 1DX III files are transferred to my workstation via their fast ethernet port which is a mechanically reliable connector and their cards stay in place.