According to Jim Recap: The Ring
Cheryl invites her new friend Lisa (Crystal Bernard) over for dinner, but after doing so, she finds out that she and Jim dated - so perhaps that wasn't the best idea. Jim isn't too happy about it, at any rate. Cheryl doesn't know that half of it.
It turns out that Jim and Lisa were engaged. But it gets better when Lisa comes over - Jim actually gave Cheryl the very ring that he used to propose to Lisa, and since Cheryl never even knew that Jim was engaged, it stands to reason that she didn't know that he had the ring. Let's just say that she isn't too happy about this turn of events.
When Lisa shares stories about how romantic her "Jimmy" was, with a horse-drawn carriage and everything, it goes from bad to worse - if that's even possible. Cheryl got a bottle of beer poured over her head, and to hear Jim tell it, he simply had an engagement ring in his sock drawer and did it a bit more spur-of-the-moment. Things aren't going well for Jimmy.
Until the end, at which point he rents his own carriage, complete with driver (well, it's Andy), and asks Cheryl to marry him all over again. He doesn't get her a new ring, but he does give her a bottle cap from the bottle of beer that her poured on her head, and that's pretty sweet. He promises that he'll buy her a new ring, too.
