The nature and spectrum of elementary excitations are defining features of a many-body system. In this study, we used a Rydberg quantum simulator to demonstrate a form of spectroscopy, called quench spectroscopy, that probes these low-energy excitations. We illustrated the method on a two-dimensional simulation of the spin-1/2 dipolar XY model. Through microscopic measurements of the spatial spin correlation dynamics following a quench, we extracted the dispersion relation of the elementary excitations for both ferro- and antiferromagnetic couplings. The ferromagnet exhibits elementary excitations behaving as linear spin waves, whereas in the antiferromagnet, spin waves appear to decay, suggesting the presence of strong nonlinearities. Our demonstration highlights the importance of power-law interactions on the excitation spectrum of a many-body system.