How do I get AE to open? I was messing around with the Render Engine but now when I open AE it automatically goes into the Render Engine mode - I can't open files etc.
Likely one of two things.
Make sure that you're launching the Adobe After Effects (version) application directly, and not the shortcut for the Render Engine.
Make sure that the ae_render_only_node.txt file isn't present, as described here:
Retrieving data ...