This happened to me as well when I retired. It's probably in the small print somewhere. I may have gotten a notice, and did not understand what it meant at the time.
But, essentially, the account changed - I was no longer part of " Employer X" group, but was now part of a different group. And, instead of transferring my account in kind, they cashed out investments and put them in cash. I was surprised too.
Also, they just revamped their system - I got lots of notices about that. And just got a new email that I need to create a new log in and password.
Your employer was probably paying that maintenance fee, I am now directly charged that as well.
Log in - Check what investment options you have, reinvest your money (I need to leave $1000 in cash).
Unless you currently have a high deductible insurance plan, I don't think you can add anymore to this account right now. but you need to confirm that info, as I am not sure. I also don't know, but you may have options to move your balance to another HSA manager.