My mother died and had a bank account in her name only. what must I do to close the account and withdraw the money?

You need to open an estate at the county in which she lived, and be appointed as administrator of her estate.   The Register of Wills will help you through this. You will get documents that will allow you to close out the bank account.  There are requirements as to what happens to the money, so you should probably consult an attorney, so that you don't do something contrary to what's required.