The key word here is "fell". Electrons don't really 'fall', but we
describe the process that way because it's like falling in gravity,
where you lose potential energy. Similarly, when an electron
transitions to a lower energy level, it has less energy, and the
energy it lost has to go somewhere. It's emitted from the atom,
in the form of a photon with a frequency that's directly related to
how much energy it's carrying.
