If there was no LED blink with a ML card when reinserting the battery (something that any ML-enabled camera does, even with the power switch off), it was likely a power issue.
If the camera is locked up, switching the on/off button and opening/closing the card/battery door will not work (there will be no reaction). These are all soft switches. To get the LED blink at startup, you have to remove the battery and put it back, with a ML card inserted. The
diagnostic tools can give some additional feedback, but a known good ML card is also good for checking whether the camera is alive (ML turns on the LED at startup immediately after checksumming itself, and on recent models - since 550D iirc - Canon bootloader turns on the LED while loading ML from the card).
If the camera is not locked up, you also get the startup LED blink on actions such as opening closing the card/battery door, or (un)twisting the lens, with the power switch off. In other words, that happens if (and only if) the camera was powered down cleanly.
If the firmware update was stuck at Loading, it was most likely an issue reading from the SD card. Formatting the card from the camera (important) and copying the FIR afterwards should help getting past this step. However, I doubt upgrading and downgrading the firmware will solve anything, and I wouldn't advise trying it when you suspect a power issue.