|Many BIOS setups (especially on OEM computers) don't have any BIOS settings for on-board sound even though such a device exists, so creative and other sound-card manufacturer's had to create drivers which disables the on-board sound chip (where present) to prevent a resource conflict.|
Your on-board sound chip may have been faulty for some time, but only now have you discovered this. I've seen and fixed many an old PC for friend's where the integrated sound chip had packed up - the only fix being of course, fitting a PCI sound card.
Sometimes it's just a case of re-installing the motherboard's chipset driver, then the on-board sound driver, from the mobo maker's website. But if that doesn't work you'll have to put the SoundBlaster card back.