You have fouled up the pixel aspect ratio or the comp size somewhere but since I don't see any info about PAR in your Media Encoder settings or know anything about your composition I have no idea where the problem is.
Basic rule, Make all your comps using standard presets that use square pixels, use the appropriate preset in the AME.
Watch the last video in this series for rendering basics: Basic Workflow
First, thank you so much for answering so quickly Rick, I really appreciate it.
It seems you're right, I didn't choose square pixels. But now all my work is completely distorted. I spent days animating with puppet pins and all, but now everything has moved out of place and I don't know what to do.
Is there a quick way to scale everything so that I can have my video back the way it was again?
Thanks so much!
Where is your error? If it is in the comp, then what are the comp settings? Again, it takes details to get a useful answer.
So my pixel aspect ratio was set to "HDV 1080/DVCPRO HD 720 (1.33)". Frame rate is 23.976.
I have now set it to HDTV 1080 25 as the preset, and although I believe it's rendering fine, all my puppet pins are not in the right position anymore.
Here's the proper workflow. Take your comp with the incorrect PAR and just nest it in a standard HD 1920 X 1080 square pixel comp and make the frame rate match. AE will automatically make pixel aspect ratio corrections to your footage so the images will not be distorted but, they may look different if you were not working with PAR correction turned on in the comp window. Scale up your project to fit and you're ready to render.