To add to @Tanith Rosenbaum's answer, the energy can be lost to other energy levels by emitting a photon to that other level, say n = 4 to 3. The total energy is conserved, thus the energy of atom in n=3 + photon is the same as that of the atom in n = 4. Emission (fluorescence) is a spontaneous process, and the lifetime of an excited atom can vary widely, typically from nanoseconds to seconds. The reason for this depends on symmetry and angular momentum of different energy levels (generally called selection rules).
To loose energy in any other way, say by collision with another atom, then that atom has to carry away the difference in energy just as the photon did. Note that a collision can also add energy by the same reasoning, as can absorbing a photon so the atom will now contain even more energy. In a more detailed examination, properties such as symmetry and angular momentum have to be considered.