Call Roto-Rooter. They will put a device (a snake) in there that travels down the length of the pipe. It has rotating blades that cut the roots. In some cases you will then you need to kill the plant. Or you could buy the device yourself, but it is expensive, and for using it one time, it is cheaper to get Roto-Rooter. Other options are: you could dig up the pipe, and chip a hole it, and remove the roots yourself, then put a patch over the hole. Buy a sewer tape, run it down the pipe, assuming that you have a clean out a hole. The sewer tape would cost as much as Roto-Rooter and it's a messy job.
