A quick way to do what you are asking is as follows:
Select the column say column C (click on the "C")
that already has your data in it.
Press the F5 function key - from dialog box click on Special... then click on
BLANK, OK.
The used range (only) of Column C will be selected (highlighted). The first
blank cell will become the activecell.
In it enter 0 (that's zero) <<DO NOT PRESS ENTER,,,
Instead Press CONTROL + Enter (simultaneously).
All previous blanks should now = 0
HTH