If you have locked a record, the best way to unlock it is to finish editing it. If however you started editing it and closed the window, you can unlock it by clicking the 'Unlock' icon in the record's option column.
If you didn't lock the record, it will either become available after the user who locked it has finished editing it, or within 15 minutes. The SSI Online Administration System unlocks all locked records every 15 minutes.