I have an iBook G3 and it had Mac OS 10.1.5 and Xubuntu installed.
When I would turn on my computer there was a screen asking me to
click l to start with Linux
click x for Mac Os
click c for CD

Than one day I upgraded my Mac OS from 10.1.5 to 10.3.Now 10.3.9.
But when I open the computer now the dual boot screen is gone.
It simply start up with Mac Os.
Why did that happen?Is Xubuntu still installed on my computer?

It sounds like the upgrade re-wrote your MBR. Xubuntu is probably still sitting there in its partition, waiting to be called by a bootloader.

When you originally installed Xubuntu, how did you do it and what bootloader did you use?

Install a new bootloader. Pop in the cd and get GRUB or LILO reinstalled!!

Google super grub boot disc. This is a handy boot disc that will install a grub bootloader and repair your mbr.

If the iBook uses BIOS, then do what the guy above me said. I don't know, if it uses EFI(which I think I know doesn't) then install rEFIt.

That's a PowerPC machine, so no BIOS, EFI, MBR etc. However, an overwritten/deactivated bootloader could very well be the problem. I don't have a lot of experience with PowerPC Macs, especially running Linux on them, but my guess would be that you could boot from the Xubuntu disc and reinstall the bootloader - give it a go and see if there are any repair or rescue options.

Also, you could try holding Option during startup to get a boot selection menu, but I don't know if the firmware will recognise the Xubuntu partition.