Visual Basic 2008 help

Hey everybody,
My homework on visual basic 2008 is on 'if else' statements.My program works fine but my lecturer has given a bonus question,He wants the the program to run as follows...when the user leaves the textbox empty and presses the button,an error comes up,which he doesn't want the error to show.Instead he wants a message box to pop up telling the user that the textbox cannot be empty and he should retry or cancel,he says this should be done in an 'if else' statement.

hope this makes sense
thanks

You program would take the basic form of:

If button is pushed and textbox is empty
Then display message box saying "Textbox cannot be empty"
Else continue with program