Gym Date

I asked my date to meet me at the gym today. She didn't show up. That's when I knew we weren't gonna work out.

Explanation

This is a pun using the phrase "work out" in two different ways. The first way is the literal meaning of exercising at the gym, and the second is the figurative meaning of a relationship succeeding. The punchline plays on the fact that since the date didn't show up to the gym, the relationship is also not going to "work out."