Can someone regain a Beginners Mind when it has gone?
In the Mahasi tradition we don't have the idea of beginner's mind, just a continuous refinement of the process. Many times throughout my practice, I've learned that some phenomena or thought process was just that—where previously I mistook it for a deeper reality. And I continue to experience these refinements.

But to attempt to answer your question, try not to hang on to the desire for Beginner's Mind. It will come back on its own. In addition, I would say be gentle to your mind and body. Those particular helped me the most when I was struggling in my practice. Maybe it will work the same way for you, in that being gentle with yourself may allow the Beginner's Mind to return when you least expect it.