I am sorry for your loss and your hardship.
I recommend that you contact an attorney who deals in probate issues in your area to represent you before the court. Tell the attorney about all of these problems. Bring the will and any other information you have about your mother's assets. Don't worry if you can't find it all to bring to the meeting. Go with whatever you have. The attorney will help you to sort this all out. In addition, tell the attorney about your medical problems and the attorney will advise you about what you need to do next.
Don't ignore the letter. Go see an attorney right away!
This answer does not constitute legal advice and does not and is not intended to create an attorney-client relationship. The law may vary depending on the state in which you reside. It is intended only to give some direction in which to seek assistance.
Circular 230 Disclosure: Pursuant to recently-enacted U.S. Treasury Department Regulations, I am now required to advise you that, unless otherwise expressly indicated, any federal tax advice contained in this communication, including attachments and enclosures, is not intended or written to be used, and may not be used, for the purpose of (i) avoiding tax-related penalties under the Internal Revenue Code or (ii) promoting, marketing or recommending to another party any tax-related matters addressed herein.
You should certainly contact an estate attorney right away. In New York State, an estate attorney handles the majority of the probate proceeding.
The Court cannot take away your right to letters testamentary without giving you notice and an opportunity to be heard. I suspect that the Order that you received is giving you this notice. If you fail to comply, you may be deemed to have renounced your right to letters testamentary. If you have any additional questions, please feel free to contact my office.