No power to a branch circuit
Updated based upon comments.

This type of failure could be attributed to any one of the following:


The GFCI tripped.
The GFCI has broken and stayed in a fault off state.
There is the possibility of there being multiple GFCIs in series on the circuit and one that you have not located yet has tripped.
Something changed in the area of the abode such as drilling holes into walls or pounding nails or driving screws that have resulted in damage to a wire in the wall.
Some part of this particular power circuit was wired up with poke in wire terminations on the back of the receptacle(s). One of these connections has failed.


Note that item number 5 has a pretty good chance of being the problem if you cannot account for any of items 1 to 4. Replacement of outlets with newer better quality ones and using the side screws is the much preferred method of securing the wire connections.