the cancel proprity is
vb2
so :

Dim Result1 As String
try1:
Result1 = inputbox("What is your name ?","","")
If not Result1 = vb2 then
If not Result1 = "" then
'All is ok ..let's continue
else
a=msgbox ("you leave the box blank !",vbcritical,"ERROR")
goto try1
end if
else
end
end if