I’m eventually going to post a topic about how in the last several years it’s become harder and harder to design “one-size fits all” kernels. The reason I’ve become interested it this is that my sacrificial laptop–the machine I use to experiment with–has wildly different boot times depending on which kernel version I use. For example, any kernel in the 4.4 series will boot up in 23 seconds; any kernel in the 4.15 series boots up in over 7 minutes. (For this reason I can’t use Mint 19 on this machine–it takes 20 minutes to boot and is unstable when it does.)
The reason I bring up this seemingly unrelated topic is that your boot-up weirdness started after installing a new kernel. And while major kernel regressions have become much rarer lately, small issues seem to be common–especially on older machines such as mine. (Though your machine is newer, I think?)
The problem here is almost certainly within the latest kernel. And while it is always possible to repair it, for all practical purposes it really isn’t. The bottom line here is that the easiest way to be rid of this graphic weirdness is to stop using this kernel and use the previous version instead. Here a couple of ways to do this:
1-Since a Grub screen now appears, hit the down arrow to “advanced settings” and select the previous kernel (4.15.0-55). The machine should be back to normal on the next boot. If it is, the next step is to make Grub run invisibly. (While there are doubtless ways to do this in the terminal there is an excellent graphical app called “Grub Customizer” which allows you adjust everything Grub, including whether or not it appears. Check out this article by Abhishek: https://itsfoss.com/grub-customizer-ubuntu/ to learn a little more and for instructions on installation.) Basically the offending kernel is still installed, but it’s not active.
2-If the problem persists, it seems likely then that Cliff is partly right–the problem is with the graphics card but it’s a software problem. As I understand it–and please anyone correct me if I’m wrong–what we like to think of as “proprietary drivers” are now elements of “dynamic modules” as Linux moves down the road towards being a truly modular OS. These are rebuilt each time you upgrade the kernel using the previous version as a template. So if the latest kernel trashed your graphics all future versions should inherit the same bug. So the only easy way to fix this is remove all traces of the new kernel. If you’re using your Timeshift simply restore your system to a point just prior to installing this kernel. Kernel 184.108.40.206.60 will, of course, immediately come up again among your updates. Don’t install it. Wait for the next kernel and install it instead.
Hoping this might help,