What is a silent ban?
A silent ban is - alledgedly - when someone has failed a doping test or other regulations and told they can't compete for a specified time period, but also that this will be kept silent so the general public won't know about it. In other words, you can ban someone for doping offences without ever letting the fans know he doped. The banned player will then presumably claim to be injured to have an excuse for not playing.
This is just speculation, noone can be sure something like a silent ban has ever existed.