I have been seriously cyberbullied in the past. The worst was in 2008 when a group of people I really thought were my friends ganged up against me and plotted to have my online accounts hacked and destroyed. They used my best friend at the time to trick me into handing over my passwords, which, because I trusted him, I foolishly did. He locked me out of my accounts (bar one that I was able to talk to him on) and started blackmailing me telling me if I didn't do certain things I wouldn't get them back. I literally couldn't do anything and I felt powerless.
Thankfully he gave them back to me, against the other group members' wishes, and obviously as you'd expect I've never been stupid enough to give out my passwords ever since