What is the ruling on taking money from the account of a deceased person who has no heir? Is this money considered lawful (halal) or unlawful (haram)? And what should I do with the money I spent, given that I am unable to repay or return it to the bank?

The wealth of a person who has no heirs reverts to the state to be spent on the poor and needy and for the benefit of Muslims. What you both did by dividing the deceased's money is impermissible. You are both obligated to guarantee what you seized and spend it on the welfare of Muslims, or give it as charity if you despair of identifying the deceased's heirs. Sheikh al-Islam Ibn Taymiyyah said: "Whoever despairs of knowing the owners of usurped goods and deposits should give them as charity on their behalf, or spend them for the benefit of Muslims, or hand them over to a just distributor." You must spend what you took from the deceased's money on the welfare of Muslims, and repay what you spent whenever you are able. You should not take anything for yourself unless you are poor and in need, and then only to the extent of your need. You are not required to inform the bank of what happened, for fear of harm and the bank's improper handling of the money.