I am sorry to hear about your mother's death. Was she the only one on the mortgage? If not, the mortgage was due on her death.
You can only assume a mortgage if the mortgage is an assumable mortgage. Fha mortgages are assumable, others are too. Call the lender to see if she had an assumable mortgage.
You cannot do anything until the estate is settled. In your mother's will, did she leave you the house? If there was no will, the estate must go into probate to determine what her estate is and who are the heirs.
Have you consulted a lawyer about your mother's estate? If you haven't do so tomorrow.