An ex couple accidentally gets locked in the basement at a New Years Party and are forced to speak about their past, present and future.

If they are exes then (to me) the obvious time bomb is that someone is going to (re)marry very soon. Maybe the next day?
Don't know how to throw that in a concise line though-

"When former lovers accidentally lock themselves in a basement during a New Years party they are forced to speak about their past, present and an immanent wedding."

That's still off a little but do you see what I'm getting at? A marriage of one of them to someone else would be like a bomb to their relationship.

Does it sound compelling? Because I was originally thinking that but I didn't think as far as a short film..if it would stick and keeps people attention.

Yea, sort of but modern day version with a twist.