Why is the ringer sound on my iPhone not working?

My ringer sounds are suddenly not working on my iPhone 4S. All of the other sounds are working fine, but the keyboard clicks, lock sound, and notification sounds are not working. I notice that when I try to turn up the volume of the ringer, I get a blank screen like the attached photo. Does anyone know how I can fix this? Thank you! Why is the ringer sound on my iPhone not working

You probably have it on ringer silenced.