[lubuntu] Black Screen After Changing Display Resolution

After changing Lubuntu 18.04 display resolution and rebooting, I will get only a black screen after logging on.

I've tried adding "GRUB_GFXPAYLOAD_LINUX=1280x800" (or other resolutions such as 800x600) to /etc/default/grub file, following this https://askubuntu.com/questions/3515...-login-lubuntu but it didn't work.

"xrandr" will give me "Can't open display".

Does anybody have any idea how to troubleshoot this? (I'm new to Linux)