Renting or borrowing a friend's flash - same model if possible - but really, any flash is an excellent idea. Will help isolate if it is a flash issue or a camera issue. I think on the other thread, you noted that it was an Olympus flash, not Canon, worked prior to camera repair and still works with other Canon models. During the repair, did they update your camera's firmware? May not show on the invoice, so check firmware version on your camera if most recent (current???) and research date it was created which will suggest if original firmware or based on timeframe if you did it. (I just upgraded to a Sony a7rIII a couple weeks ago and still manufactured with the original firmware, so part of the setup - omitted step in official setup instructions - as discovered certain features "out the box" didn't work was updating the firmware and discovered was 2 update versions behind) Normally firmware updates are backwards compatible, so if they updated the firmware during repair it shouldn't affect Canon flashes, but 3rd party flash can't be assured.
Less likely is flash firmware. Not certain if it can (or in this case, should) be done. Never did with my Nikon SB flashes, but did with Pocket Wizard (TT1 and TT5) triggers to support newer models. Just noticed that there is a firmware update for my Yongnuo YN460 IV, but I recently switched to Godox due to non-firmware quality control issues in recent Yongnuo pre-mature failure after light use (and no drops) where appears a transistor fails and won't turn on.
Yongnou has a criticism of losing flash power over time and as a result I rotated my three when replacing following accidental damage with C being the lesser used and oldest. My last 2 "A" group suddenly wouldn't turn on when at a job site. These flashes are so inexpensive that if is cheaper to buy another one than repair - but I wasn't going to keep forking out $72 every couple of months. Finally, I heard something loose, rattling around in the on-camera controller, and before it gave out, switch to Godox.