Like Shweta has already commented, I also don't see an immediate reason to why it shouldn't work.
So please update your question with what problem you're getting.
Meanwhile you can try this:
SELECT MAINFILE.facno,GLMAST.fglhead
FROM MAINFILE INNER JOIN GLMAST
ON MAINFILE.facno like GLMAST.fglcode + '%'
AND GLMAST.fglhead='" & ComboBox2.Text & "' "