A.K.A. that frequency you were likely monitoring (except perhaps in terminal airspace) anyway, right?
Even if you weren't, there's a very good chance ATC is. From AIM § 6-3-1(h)(1):
121.5 MHz is guarded by direction finding stations and some military and civil aircraft. 243.0 MHz is guarded by military aircraft. Both 121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, FSSs, and radar facilities.
Personally, I'd probably just try another frequency from the same facility first. For example, if you're with approach, I'd look for another frequency even if it's for a neighboring chunk of airspace. Chances are the person I reach is sitting pretty close to the person I couldn't reach. If it's for tower, then I'd try a different tower frequency (North vs. South, Helicopter, etc.) or even a ground frequency.