Plumbing in bathroom:

Faucet has a knob that you push down to use the tub and up for the shower, but it is stuck in the up position.

I have tried CLR and PB blaster to no avail and also a hammer, but nothing works.

What can I try next??

Is this knob on the faucet or on the tub spout?

Sounds like the spout to me.(see image). These spouts are so inexpensive that we don't repair them. We simply replace them. There are two types of diverter spouts. The threaded type and the slip on type. To tell what you have look at the bottom next to the wall. If it has a small square opening then you have the slip on type. Pick one up at any big box store and replace your old one. Or do you have a three knob faucet with the diverter in the center? (see image) good luck, Tom