Stopping a Macro If a cell is left blank

[Guest]

Good morning,
I need to have an email macro stop and show a userform if a cell in a
worksheets is left blank.
How in the world can I do this?

 

[Guest]

Something like this...

If Trim(Sheets("sheet 1").Range("A1").Value) = "" then
UserForm1.Show
else
'proceed as normal
end if