A family of Microsoft word processing software products for creating web, email, and print documents.
I believe you may have the same issue I was having.. I am in college and when typing papers I must know how many pages I have and where each page starts as well as what the reference page looks like. Several times in the middle of a paper, I suddenly only have a display of how many words are in the document and not what page number I am on, etc. Maybe I hit the wrong key?? Any how, If you click the little icon at the very bottom of the page that looks like a little document, (if you hover it says "PRINT LAYOUT", it adds the page numbers, layout, etc. back onto the document. :)
Your last sentence explains the problem you are having: When you clicked Print Layout, you switched to a view that displays the document similar to how it will be printed. That view displays headers and footers, where page numbers normally reside.
Other views (Web Layout, Draft, Outline) are for other purposes, and they let you focus on other aspects of a document. Those other views do not display headers and footers and margins, and therefore page numbers will also be invisible.