Unclog grout PVC pipe
Assuming this happened five minutes ago: Fish out anything you can as soon as possible. Use an old coat hanger, your fingers, whatever you have to hand that's long enough. Flush the rest with loads of water as soon as possible. As long as you can get it to move along the pipe to the drainage stack, it should be flushed out and not block anything.

If it's already cured, you can either try and break it with something or replace the pipe. Anything you can get in there to break up the grout risks damaging the pipe, so you'd need to be careful. And without seeing photos or knowing what tools you have available, it's very hard to give any further advice on breaking it up.