Got my new 6D a couple hours ago, guess I won't be trying it out this weekend.
err20 means something mechanical has gone wrong. The usual method of removing the battery, sd card and lens then install everything again can usually get it up and running. I've had this numerious times on my 7D and following the instructions above have always gotten it up and running again. Also try different lenses and see what happens; my sigma 10-20 got this error all the time and when I switched to a canon lens it came back to life. Good luck I hope you get it up and running