Is it a regular GP appointment?
I am hoping you have already sorted this by now, but if not on my area if that happens, you would need to call your regular clinic/methadone prescriber. If it's the first time it's happened they would generally give you a new script. If anyone saw it happen - like if you dropped it inside the pharmacy etc then let them know as they may want to check it.
It would be very mean for them not to give you a replacement if it is the first time this has happened for you. In my area a GP wouldn't prescribe it, only the addiction clinic. My prescriber used to be a substance misuse GP prescriber but cut backs meant she had to stop thst service.
Hope you got it sorted anyway!