Cat shouldn't keep it from starting unless it was clogged.
It COULD be the main relay. Intermittent starting issues and it just so happened when you came back it clicked and you had fuel pressure to start it.
As far as aftermarket cats. I highly advise that you look at the cat to make sure it's not clogged (shine a light through it and see if you can see light on the other end) I still doubt a cat caused it to not start.
94 Civic Hatch
9.85 @ 155 mph