Can you remember what was in it?
The codex lists the individual items you've discovered, so if you know if it has an earthlike, or what sort of star it was, you could find it in there.
Otherwise, it'll depend on whether you're sync'd up with Inara or EDSM.
Inara has a flight log (in the commander / personal logs / flight log)
EDSM I don't know so well, but I understand it's the best for logging discoveries.
Worth mentioning that if the '1st discovery' was recent, I was getting 1st discoveries for everything I scanned a few days ago, although this was obviously a glitch, as they were all earthlikes in the bubble.