Somehow I have turned off my inventory screen....probably hit some button by accident. What do I hit to get it back. And this game doesn't seem to have a help file

Click on the icon that shows your currently equipped weapon.

Hit Esc then options and keyboard settings then you can see all the commands and change them if you have another arrangement you prefer. By default, if memory serves as I changed it, it was the right mouse button. The only one of the commands that I don't think you can remap is the hot bar which is F1.