This post is describe how to handle problem with "Missing Event Receivers".
You received for example such message inside PreUpgrade Report.
Assembly = ListEventReceiver.TravelPortal, Version=188.8.131.52, Culture=neutral, PublicKeyToken=c033f84e3e5850b7, Count = 6, Status = Missing.
You need run stsadm.exe command for locate where this event receiver is used.
cd C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\BIN
stsadm -o enumallwebs -includeeventreceivers > c:\migration\eventRec.txt
this command create text file that contain all site references for event receivers in your
Sharepoint Enviroment, but isn't enough for this file you can't know where specific event
receiver is attached.
For this purpose you can use Event Handler Manager, that can download for codeplex site.
This is very helpful tool for management event receivers
By using this tool you can locate all lists and libraries for specific site, that contain missing
event receiver, you find it inside created text file, after this you select all of them and press
button "Show Registered Event Handles". After this you need select this missing event receiver
inside "Registered Event Handlers" and press "Remove Selected Event Receivers".
How you can do for all missing event receivers and solve this issue.