Here is what to do next time:

1) As channa wrote above, check-in online before you leave for the airport. Even if you can't print your boarding pass, you can tell them you are already checked in for the flight and ask them to "reprint" your boarding pass for you.

2) If that is not possible, take the boarding pass issued to you for the afternoon flight and immediately go through security and right to the gate for the flight you really want. Speak to the agent at the gate, you often can get back on that flight if there is space available.