Sometimes Campus doesn't let you change the values on drop down (selection list) fields. This can happen when trying to change years, schools, schedules, sections, etc.
The way to "unfreeze" the drop downs is to refresh/reload the page. F5 works in IE on a PC, Command-R works on Safari on Mac.
Unfortunately, what seems to be happening to you may not clue you in to what is wrong. You might think you can't do grading, or don't have grading tasks, or any one of many different things you might be trying to do.