Is the knob still tight? If you turn the pump on it stays together?
The rod it connects to threads into the bottom of the filter, so it's possible the whole thing has come loose, but the lid should still come off.
The 'popping' sounds a bit more like the threads are just crossed. If that's the case you'll first need to order a new knob and tie rod, then you'll need to get creative in order to get the old one off. Clamping on to it so that you can apply a lot of upward force while you turn may at least give you enough torque to turn the bottom threads.
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