The Flashing Question mark on folder means the firmware can't find your boot OS.
It could be it just "forget" where it was, hold the option key down while rebooting, you will enter the Startup manager and then can select OS X to boot from.
If this works and you get in, set the Startup Drive in System Preferences to your boot drive and reboot to test.
If this doesn't work, your drive or a cable or the Mac may be at fault.
The second screenshot you show, shows the EFI being loaded and then it's trying to load the first of your third party (or Apple if you have no third party) kernel extenssions that starts the process of OS X booting up.
Since it can't fnd the drive, it can't the kerenel extensions to load obviously.
Go through this check list
Thanks for your speedy reply!
I tried holding down option (and then shift following your list of steps) with a bootup cd in. It recognised the cd, giving that as the only option to boot from, but that resulted in the same series of symbols. I suppose something else must be wrong and I'll have to get into the machine.
I've tried some of the other steps but there's a lot to go through.
It recognised the cd, giving that as the only option to boot from,
Mean you drive could be dead or another hardwre issue.
...with a bootup cd in. It recognised the cd, giving that as the only option to boot from, but that resulted in the same series of symbols.
You could have used a wrong CD that doesn't match the machine, it's scratched or dirty, or you have a more serious hardware issue.
It's looking like a trip to the shop.