Prevent user from closing worksheet if cell is blank

I'm not sure how to do this and hoping someone can help. I have a worksheet that is used by multiple users. For example, in Row 2, User 1 needs to enter data into the first five columns (A-E) and User 2 needs to enter data into the next five columns (F-J). I've protected each of the areas so that each User can only access their respective area.

What I would like to do is have User 1 complete all cells in their range (A-E) for a specific line (i.e., when something is typed into a row, that row needs to be completely filled out (A-E). Rows below the entry line (blank) do not apply. If they try to save and exit the file without doing so, I need a message prompt to state they need to complete their fields.

My second issue is that this is a worksheet that has been used for many years and there are several cells in the rows that may be blank. Since we cannot backfill this data, the cells need to remain blank. So, the range this code would be applicable to would be the columns (A-E) and the rows from current going forward (i.e., beginning at line 1000).

I'm a beginner in writing vba code and everything I've attempted has failed.