Yesterday I upgraded the Ubuntu from 16 to 18. Before I upgraded, I think I removed several packages. After the upgrade, I realized the GPU that was connected before is not recognized anymore and the resolution is just the default. My monitor is supposed to have 4K (3840 x 2160) resolution, but currently what I have is the default (lowest) resolution. My desktop cannot even recognize it. These are the two things I need your help. Thanks!