The lens and memory card were then removed, and the main battery was pulled out for a few minutes, but after re-installing the battery, it still failed to turn on.
I had been using the camera out in the sun on a hot 37 degree day, and thought that potentially the heat had affected it. After cooling the camera down for a while in the air-conditioned coolness of my car, it still refused to turn on.
I gave up on the 350D for the time being, and had to resort to using a single camera body for the rest of the day. I like to have a wide lens on one body, and a long lens on the other body, allowing a wider variety of focal lengths without having to swap lenses. Being reduced to a single camera body was an inconvenience, but it wasn't a show stopper, and this experience emphasises the need for a backup camera body when doing an important shoot!

from a Canon 350D DSLR body
While googling for information on the problem, I found this thread on dpreview.com that described the same symptoms.
In that thread, someone suggested the following sequence as a potential cure for a lockup:
- Turn off the camera
- Remove both camera batteries
- Close battery cover
- Turn on/off switch to "On"
- Let camera sit for at least 15 seconds
- Turn on/off switch to "Off"
- Re-install both batteries and turn camera on
Setting the on/off switch to "On" with both batteries removed appears to be the key step that I had previously missed. I'm guessing it's required to ensure any residual power in the camera's circuitry is completely discharged, thus allowing the camera to completely reboot.
(Closing the battery cover is also important, as the camera's on/off switch is disabled if the battery cover is open.)
Note that removing the date and time battery will predictably reset the camera's date and time. However, it won't cause the camera to lose any other configuration data, such as custom function settings.
Have you ever had a camera fail at a bad time during a photo shoot? How did it fail? Did you have a second camera body with you at the time? Did the failure impact the photo shoot?
My 350d suddenly stopped working... I tried above steps but nothing helped.
:(
Any other suggestions ??