Hi,
Have you received a silence in Diablo 3 or another Blizzard game ?
If so, there is currently a bug that prevent silenced players from starting or joining a Diablo 3 game; there is no workaround and the last word on this issue is that Blizzard is looking into it. No ETA for a solution either.
If this is your first silence, it is usually of a short duration… 24 hours usually. You should be able to start or join a game once the silence period has ended.