Reboot the machine while holding down Cmd-Opt-X-O. This should force the Classic to boot from ROM, and then you'll (hopefully) be able to see what's on the HDD. It could be that the drive is failing and causing an extension to not load properly -- if it's got the original Quantum or Conner 40MB HDD, this is definitely something to look into.

Is the sound working? Is there any goo or green "rust" under the caps?

-lee