Device or computer?
Do you have access to another network where you can try to log in? That has worked for several people.
Try a restart.
Try a forced restart.
If necessary
Device or computer?
Do you have access to another network where you can try to log in? That has worked for several people.
Try a restart.
Try a forced restart.
If necessary
Why can’t I login to my iCloud when my password and username are both correct?