Signal processing for undersampled signal
Combining @JRE's comment and @Nikita's answer: if no anti-aliasing filter was applied to the signal, and no other frequencies alias to 20 Hz, then the 250 Hz component you're interested in will appear at 20 Hz in your sampled signal. See https://en.wikipedia.org/wiki/Aliasing#Sampling_sinusoidal_functions for more information on how this works.