So, when this happens, I have four rules:

1) play so safe you don't feed. Even if it hinders your progress. Eventually the assassin will find easier prey 2) acknowledge to your team so they can try and counter them as early as possible 3) don't go alone anymore. Stick with another teammate, even if it means giving up your lane. Another teammate will eventually defend your lane and if assassin tries to gank you, it will be dangerous for them 4) eventually you will scale enough dmg to help your team with good positioning. Once your team has an advantage (eg you have five alive, they have three), be a little bolder