Is there a snippet of code to force a person to complete a field?

i have a combobox ("CBRoot") with several root causes and i need to ensure that if the user does not fill in the box that they are taken back to the original userform("FrmUpdate" with it still containing all the data they have inputed) then they can enter a cause and then the form fills in the relevant cells once they select the"add" button?

i'm sure it's easy i just can't see the wood from the tree's at the moment?

Any help is greatly appreciated and thanks for reading my thread people