Why is my Time Machine icon now showing a constant exclamation point?

I have Time Machine on my MacBook Air OSX. Since I did not need backups done hourly, I downloaded Time Machine Scheduler and had problems. Suddenly I was seeing the Time Machine icon on my desktop consisting of a cirlcle with an explnation point. I don't know what's wrong and I can't find out how to fix this problem. I did uninstall the Scheduler, but still the issue exists. Help!