How to lock cells which are automated filled - excel

Question

I want to lock the cell after entering a value in it. When I change the value on sheet2 A1, the value should still be locked in B2.
When I enter "3" in Sheet2 A1 the number 2 should till be there.
Here the code I already have:
Private Sub Worksheet_Change(ByVal Target As Range)
Sheet1.Unprotect "1234"
If VBA.IsEmpty(Target.Value) Then
Target.Locked = False
Else
Target.Locked = True
End If
Sheet1.Protect "1234"
End Sub

Answer 1

My first answer was assuming that cell Locking referred to the Range.Locked property but now I understand that was intended to refer to the cell values and preventing them from recalculating.
There are a few techniques that can be used to prevent cells from recalculating. The easiest would be to just change any formulas into a static value like:
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
Target.Value = Target.Value
Application.EnableEvents = True
End Sub
This event will get rid of every formula after it calculates its value the first time. Every cell will just be whatever value the user enters or whatever value the formula calculates the first time.
You can limit the range that the event will operate within by using Intersect like:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim OperationalArea As Range, AffectedArea As Range
Set OperationalArea = Me.Range("B2:F10")
Set AffectedArea = Intersect(Target, OperationalArea)
If Not AffectedArea Is Nothing Then
Application.EnableEvents = False
AffectedArea.Value = AffectedArea.Value
Application.EnableEvents = True
End If
End Sub
This would limit the event to only change cells within the area "B2:F10" as defined in OperationalArea.
If you don't like this idea you can try messing about with Application.Calculation but it gets very messy when you have multiple sheets or workbooks open.

Answer 2

When you do Worksheet.Protect, every cell is locked by default. You need to specify which cells should still be editable (not locked) after protecting the sheet. In your case, you need to specify that every cell except your chosen few are not locked.
The second important note is that Worksheet.Protect has an option UserInterfaceOnly which when true, allows macros to continue editing cells without needing to unprotect the sheet first. This means you can leave the sheet protected and just unlock or lock cells as needed.
Here is an example of how to unlock every cell except A1 on Sheet1:
Sub Example()
With Sheet1
.Protect Password:="1234", UserInterfaceOnly:=True
.Cells.Locked = False
.Cells(1, 1).Locked = True
End With
End Sub