One reason is that the author of a workbook or add-in locked it from viewing to prevent you from seeing the code.
You protect a project via the VBAProject Properties dialog, Protection Tab.
If this is the case you will need the password.
If the file is on a network, try to unshare it
To unshare it to open the VB project.
You can always share it again if you want to.
The Hex editor may not help you to unlock a VBA project that is Unviewable. Hex editing is more complicated for unviewable projects compared to plain VBA password removal. You risk corrupting the workbook if you edit bytes without knowing its inner workings of the file.
Unviewable projects are made for 2 reasons:
1) When a workbook is Shared. To remove a shared password use: