IsNotNull In Query

[dfwcomputer]

hi guys,

I have this query which looks up machines by there serial number, It brings up a pop up box saying "What Serial Number" which is where yopu type in the serial number of the machine you want to view.
But i would like to display an error message if no serial is entered, is this possible.

SELECT tblMachines.MachineID, tblMachines.ModelNumber, tblMachines.VinNumber, tblMachines.WorkOrder, tblMachines.SerialNumber, tblMachines.EngineNumber, tblMachines.RegoNumber, tblMachineDetails.PartsID
FROM tblParts INNER JOIN (tblMachines INNER JOIN tblMachineDetails ON tblMachines.MachineID = tblMachineDetails.MachineID) ON tblParts.PartID = tblMachineDetails.PartsID
WHERE (((tblMachines.SerialNumber) Like [What Serial Number?] & "*"))
ORDER BY tblMachines.VinNumber;

Thanks