How do you get Excel to display an error message if a cell is left blank?

I have a workbook with multiple sheets and what I want to do is for Excel do
to is look at a certain range and display an error message if a cell within
the defined range is left blank. I want this message to appear when someone
tries to advance to the next sheet.

For example: In Sheet 1 have a row of questions cells A5:A50 and a place to
answer those questions B5:B50 and when I am finished with answering the
questions I have to click on the next sheet, but in case I forgot to answer a
question (leaving one of the B cells empty) I would like Excel to display an
error message that says something like "you have left a required field empty,
are you sure you want to continue"

Thank you in advance for your help