"you have received a reward"

you probably got a summer icon....check your unlocked summoner icons

if it was from riot games most likely that new icon

If not then im stumped best bet would be to submit a ticket asking what was given out and go from there

Could be the new summoner icon, yes. Though Riot also recently enabled those notifications in the new client, so you might also see some notifications for rewards you had received in the past (just a one-time thing since they enabled it).

Check service statuses. Most recent one said that they fixed the problem with notifications but that players may be receiving a notification for things they have already received.