Selected Answer
You don't need VBA for this. You just need to lock the cells.
Select the cells you want to be able to alter by hand and right-click them > Format Cells... > Protection tab > uncheck Locked and hit OK.
Then go to the Review tab and click Protect Sheet and enter a password to use to unlock everything.
This allows cells to contain formulas and functions and allows them to update but doesn't allow users to edit the formulas or functions.
OR
After re-reading your question, it sounds like you may just need to turn calculation back on: Go to the Formulas tab > Calculation Options > and make sure there is a check next to where it says Automatic.
Update
Tutorials from us that explain how to secure data and protect worksheets.
The second link is a macro like you wanted.
Allow Only Certain People to Edit Specific Cells in Excel
Hide Formulas in a Worksheet and Prevent Deletion
The first link basically just explains in more detail what I already said. In addition, when you go to protect the cells, you might want to make sure that there is no checkmark next to where it says something like "select cells" or similar.