She could have over extended her jaw, by trying to fit something in it too large.
Or injured it fighting.
This Q&A at this site may help. Fish: Kribensis with lock-jaw, cichlid research, hospital tank
If the suggestion listed there does not help, I have also read you can feed her by a syringe filled with vitamin soaked food and a broad spectrum antibiotic, and hope it works itself out.
I asked what her diet was, because a staple diet of krill has been linked to lockjaw in predatory fish. This is only because it causes vitamin deficiency.
There are certain internal parasites and bacteria that can cause lockjaw in fish.
Hexamita, which causes hole-in-the-head, can cause lockjaw.
And Fish TB
The 2 conditions listed above usually have other notable symptoms though. It's likely she has injured herself, and if her jaw is broken, she may never recover.