Answer: Failed to show up to work over the weekend
Just before Peter went to the appointment with the hypnotherapist, he was trying to stealthily leave work so that Lumbergh couldn't find him and ask him to work over the weekend. Lumbergh did find him however, and told Peter to come in the next day at 9, and go ahead and come in on Sunday too.
At the appointment, the doctor put Peter into a state of total relaxation, but the doctor had a heart attack and died before he could bring Peter 'back'. Peter stayed in his relaxed state, completely changing his outlook on life and his work. The next morning, he ignored his alarm going off and ignored all the phone calls coming in from Lumbergh asking Peter where he was. When Peter finally got out of bed, he had 17 messages on his answering machine, from Lumbergh. He deleted them all and didn't go in to work.