When leaving an iced tea or bar of ice cream on the counter while going into the fridge to get something else, when you come back out it may have disappeared into a pool of watery tea or scoop of melty ice cream - what happened to make it disappear?
Simply stated, the answer lies within exposure of ice to heat. Warm air temperatures and sunlight caused the ice to change its state from solid to liquid state and eventually reach its melting point (32 degrees Celsius for water).
Once ice reaches its melting point, its energy begins to dissipate as its molecular structure loosens to allow its molecules to move more freely. This is what happens when left out on your countertop during the heat of summer or exposed directly to sunlight - once its molecular structure cannot hold together anymore it breaks apart into smaller particles that move around more freely than previously possible.
As the molecules within ice loosen and move more freely, they also absorb more heat from both sunlight and ambient air sources - this heating causes melting to accelerate until all ice has completely disintegrated into liquid state.
Most materials have a fixed temperature at which they melt; in the case of pure crystalline substances, this point is known as their melting point; for impure or non-crystalline substances however, identifying exactly when melting occurs can be more challenging; generally amorphous substances tend to melt over a wide temperature range while crystalline substances usually do so at specific points in time.
Frozen liquids generally freeze at very low temperatures, though there are exceptions such as alcohol which freezes much colder and chocolate which freezes at a slightly higher temperature.
Some substances don't melt because they simply lack enough energy. Most complex organic substances, like paper, don't have enough stability for it to take enough energy to break apart (although they will burn quickly if exposed to flame). Other chemicals, like nitrogen triiodide poison gas, are extremely unstable and will explode upon receiving any added energy source; nearly all metals (such as dense tungsten) can also be liquefied with sufficient heat in a controlled environment without decomposition occurring.