Had the same issue! For me, it was because I upgraded 9.10 -> 10.04 and kept my modified menu.lst file. Running uname -r returned 2.6.31-14 even though I was running Lucid. The latest updates messed up my nvidia drivers. I ended up manually editing the kernels that were being booted in my menu.lst file, updated grub, and uninstalled, reinstalled the nvidia drivers and it worked.
Had the same issue! For me, it was because I upgraded 9.10 -> 10.04 and kept my modified menu.lst file. Running uname -r returned 2.6.31-14 even though I was running Lucid. The latest updates messed up my nvidia drivers. I ended up manually editing the kernels that were being booted in my menu.lst file, updated grub, and uninstalled, reinstalled the nvidia drivers and it worked.