Start calmly and specifically: "I have an appointment for assistance to connect the number..., waiting at the designated location from..., I need to get to..." Contact the carrier worker, information service, shift manager or driver.
If you are travelling with your partner, divide the tasks: one stays with the passenger, the other is looking for help. The passenger knows where he's waiting, how long he's been around and how his partner's gonna call. My partner's going back to exactly the right place or he's calling first.
Write down the time, location, connection number and what happened. Write the complaint to the carrier in substance.
Source: Your Europe