Lock out data entry in a cell

[DShaver]

I need to lock out data entry in a cell based on whether text (from a data
validation list) is entered in another cell.

I have a column labeled Amount. I want to lock each cell and only allow
data to be entered if the cell above it has an amount entered. The first
cell will be open for data entry. I'm doing this so a user has to go through
a form sequentially line by line and can't skip a line.

 

[DShaver]

Clarification - the first cell will be locked and only allow data to be
entered if a value from a drop down list is selected in a cell above. Each
cell below the first cell is dependent on whether a value is entered in the
cell above it.