Won't play back properly, as if there is an invisible start

I just started sketching something. I cut what I had and pasted it in at double the value with cmd-shift-Q. Then when I hit play, it started playing with just the last chord in bars that I hadn't written anything, then it played from the last bar with that chord and finally nothing. It is as if it played the one chord from where ever I hit stop until it finished all the bars. I tried undoing and creating a new document and pasting the four bars in, but it does the same thing.
I would just like to know how I did this and how to fix it.
Thanks in advance