Try starting MSFS as an administrator.
If that doesn’t help, this is a link to the article in the sim “Support” section, found at the top of each page of the forum.
https://flightsimulator.zendesk.com/hc/en-us/articles/360015999539-Loading-issues-stuck-on-loading-screen-long-loading-times-stuck-on-black-screen-etc-