I have Parallels 7 on my Macbook Pro. I just installed El Capitan onto my Macbook and now I can't open Parallels. What do I need to do?
You're going to have to upgrade to Parallels 10 or Parallels 11. See this Parallels knowledgeable article: Parallels Desktop and Mac OS (Host) compatibility
There is a free trial demo of Parallels 11. So you may be able to convert your old Parallels 7 file in advance of paying for it.
Next time you update OS X be sure to check its compatibility with the version of Parallels you're running. Parallels is a program that often breaks with an OS X update.