If you can anticipate that need, try to divide leftovers into individual portions before you freeze it so not to run into this problem. But that doesn't necessarily mean breaking it up into multiple containers piled into your freezer.
If the freezer bag is not too full, I've been known to "pinch" or section off portions of the bag before it freezes solid. Those big snack clips make great dividing tools if you clamp it on from the outside. That way you can more-easily break off chunks along the frozen perforations you created.
Depending on how wet the food is, you can also try to divide portions between layers of wax paper before freezing. The layers don't have to separate the food completely. Adding two layers of wax paper between the portions as you add them to the bag is typically enough to let you slip a knife in there and break them apart after they freeze solid.
But barring any of these pre-preparation methods, you can pop the frozen mass in the microwave and start defrosting it just enough to loosen it up a bit. You don't want to completely defrost a giant block of food you intend to refreeze; it will turn to mush or get that warmed-over taste many times over (yuck). But microwaving the block just long enough to get past that "solid mass" stage will usually loosen it up so it can be broken apart with a knife. A block of food just below freezing is much easier to break apart than something that has been sitting in the deep freeze.
