I don't think there's a very good way to trace PVC pipe. Since it is non metallic, metal detectors will not work. About the only way I know is to dig out around the sprinkler and see which way the pipe heads. Then use logic (which may or may not work) and assume the pipe would have been laid with 90* angles. Then dig some test holes to try to find the pipe. That's probably too much work just to be able to use the existing pipe, so you might be better off just to lay a new line and forget about the old one.