A shower problem, please help?

In my bath I have two nobs and a spout that are all separate. on the spout I have a little nob on the top to pull for my shower... Very typical. Anyways, today I noticed that when I pull it up, the shower wont start. It feels like something on the inside is detached. There is no screws or any type of hole on my tap. How can this be fixed? Ultimately, I would like to do it myself. Please help