Received an iCloud Backup message and now my iPhone 4s is useless

A box popped up entitled "iCloud Backup", stating that "This iPhone hasn't been backed up in 10 weeks. Backups happen when this iPhone is plugged in, locked, and connected to Wi-Fi."

I pushe the "ok" button and it won't go away. I can't get passed this to use my phone at all. I tried plugging it into my computer so it would back up but nothing happens. Help?