If there is no heir or husband/wife, who rules the country as second in command? I'm writing a book and I'd like to know. Its sorta medieval times.

I'm guessing other family members/kin, such as uncles, cousins, etc... The Medieval times were very keen on male heirs so eldest male brother to the fallen King, eldest cousin, or something of the sort... That could be the case in your book. Either that, or it could be fought over or stay unacclaimed for.