If you used the .run installer to install, maybe you forgot to use the --dkms option for the driver to survive a kernel update. It’s better to not use the .run installer on Ubuntu though, instead add the Ubuntu graphics ppa and use the packaged driver from that.
Thanks for your suggestion that should be the correct reason.
I have just check the kernel of Ubuntu that had been updated recently.
And I haven’t installed the driver with the dkms.