One side of the tossed coin is head. So, our sample space now has only two possible coins - HH and HT but both are not equally probable.
In our sample space we can have 6 possible outcomes: (HT means Head turns up and Tail is on other side)
HH, HH, HT, TH, TT, TT all of which are equally probable.
Now, Head turns up means we now have only 3 events HH, HH and HT and only HT is favorable for the other side being Tail. So, required probability is 1/3.