I was typing on my computer and hit a key by mistake and now all of my programs are stretched out horizontally. How do I get them back to normal?

Need some more information to answer your question!
However, first of all check the front panel settings of your monitor and / or connect another monitor to your computer if available. If without success, then check the display driver and make necessary changes to rectify the fault.See windows help to learn more.

May be u have hit some display hotkeys like ctrl+alt+'some key' that support different dimensions of display.