If you reboot your PC after a Linux Mint version update (e.g. 17.3 to 18) and find things to be a bit wonky, you’ll need to power cycle the PC. The reason for this is there may have been a kernel version update, and Linux Mint always boots into the latest kernel installed. However, it does this only if the PC is power cycled. If it isn’t, the new Linux Mint build will boot using the old kernel, which causes the problems you may be experiencing.
You’d think this would be in the official version update instructions, but this is Linux, and so caveats are left to unlucky users to discover for themselves.