Video is cut in half when done rendering and saved

If by "cut in half," you need only half as long as it should have been, check your work area and the Media Encoder or Render Queue settings. 

If you mean your video was half-sized, you probably created custom Render settings in the Render queue or the Media Encoder. Until you become very familiar with video standards, format specifications, and compression, it is a very good idea to stick with the presets that come with After Effects. Don't customize anything unless you really know what you are doing.

The only other option I can think of is that you had the Composition set to Non-square pixels or picked a standard NON-Square format like DV-NTSC. 

Are you able to post a still image of what you are seeing on your end?  

Also, what version of After Effects are you running and under which version of macOS or Windows?

Here are some possible things that might cause this that come to mind right away:

• What are the source footage file formats?  Unexpected things can happen with low bit rate source video that holds up well enough while working in the Composition, but does not look as expected when rendered.
• Are you rendering a vertical Comp at ProRes settings?  There was an issue in an older version of After Effects where the Composition looked fine, but the picture would get squeezed while the frame size remained 9-by-16.  The work around was to send the vertical Comp to Adobe Media Encoder and export to ProRes there; however, this has been fixed in newer versions (After Effects 23.x and 24.x should be fine).
• Are you sending the Comp to a version number of Adobe Media Encoder that does not match the version number of Adobe After Effects?  There was an issue with picture being incorrectly offset, causing the left side to show on the right.  To address this, make sure you're sending Comps from After Effects 24.3 to Media Encoder 24.3 or 23.6 to 23.6.

Did you use an anamorphic lens when you shot the video? You need to set it as it is streteched.