Query the agent again. Mention that they offered on X manuscript in 2014.
Dear Ms. Lastname,
I'm not sure if you remember me, but you offered me representation in 2014 for my novel An Awesome Horse about a character who finds an awesome horse and overcomes great obstacles with her horse.
I'm back to querying, and I wanted to reach out to you first with my new novel, An Incredible Duck.
Ducky is a teenage mallard with a problem....(rest of the pitch)
They understand business decisions. Agents get turned down all the time--not just by writers, but also by editors, editorial boards, and even other agencies. It's more weird if you don't mention that you previously had an offer than if you do.
Good luck!