As others post, it would be well to be prepared with the proper hand tools for obtaining wood.
Freshly cut wood wont burn well, though it is better than nothing.
Remember though that in any widespread emergency there will be intense competion for the available wood, and the owners of the trees might well shoot people stealing them.
It would therefore be well to have several years stock of firewood.
If utility service is available, but you fear it will go off long term, then use gas or electric heat whilst available so as to conserve your wood until it is needed.