This is a recurring problem for some users and seems to be tied to one or more of three things: 1) addons, 2) running WoW.exe as administrator, and 3) your monitor's resolution.
For the time being, make sure your World of Warcraft Classic\Interface\Addons folder is empty (If you have any in there you want to keep, just cut them out and put them on your desktop for the time being). Second, right click the WoW shortcut on your desktop (Or wherever you open it from), select "Properties," click the "Compatibility" tab, and enable the checkbox for "Run this program as an administrator." Lastly, check your monitor's resolution. Try setting it to a common resolution like 1920x1080.
Refer to these two threads below for people who followed these steps and were able to get it to work. Seems like those who have weird monitor resolutions and do not run WoW as an administrator get the bahvior you guys are describing. If you're able to get it to work, you can restore your addons by dropping them back into your WoW\Interace folder again.
https://forum.nostalrius.org/viewtopic.php?f=45&t=11553https://forum.nostalrius.org/viewtopic.php?f=45&t=10739