prevent blank cell in excel worksheet

i have a work book with four tabs. i have figured out how to use the cell
validation rules, but none of those work for what i need:

if the cell is blank, i need an error message to appear (like in the cell
validation part). the cell must start of blank, and the user must enter a
value equal to or greater than 0. if the user tries to go to another cell,
they should get the warning messge to enter a vaue equal to or greater than 0.

any suggestions?

regards,

jat