Are you using LILO?
If so, then the Yast Online Update most likely compiled a new kernel for you, and the old one is marked as no longer to be booted because of the newer one available. LILO likely then still tries to load the old kernel and fails.
See for example Thread with SUSE 8.2 (forum)
However, SUSE has been using GRUB by default for a while now, so I suspect that it must be something else. GRUB probably shouldn't have this problem, though perhaps it might.
Here is a related issue on the SUSE Support Database
. You could do more searching on SUSE forums and and the support data base.
Consider using the time-limited support that SUSE offers. This requires preparing more formal information, but hey -- you paid for it.