Answer: The opportunity cost is simply anything you would have used the time of attending the concert for.
Explanation: An opportunity cost is simply alternative forgone. Something you could have done had it been you chose not to do what you did or something you could have bought if you chose not to buy what you bought.
More information are required in the question - information relating to the alternative activities you would have done if you had chosen not to attend the concert. If for instance, the concert took approximately 5 hours of your time. The 5 hours could have been used for other visits or adventures, study, sleep, cook, play some games, etc. If you chose any of these as the alternative use of your time, that would simply be your opportunity cost.