Question: I'm using Word 2010, and I'm working on a guide where contents to be added inside a table of three cells.
I have an issue with couple of pages where I couldn't insert the cursor into the left cell to add a text. It works in some of the pages, but not for the others.

I checked the Developer tab to see if any protection is active but nothing. I tried also to copy those tables into a new word, but got same result.

I couldn't find an answer online. So I appreciate your assistance.

Answer: 
This happened to me in Word 2007. For some reason the font in that cell had re-sized itself to 0.5 (just for that cell!). Click in the area of the cell and increase font size.
Install KB 4011140 from the Microsoft Website. It worked for me.