For some bizarre reason, Enterprise Manager has a nasty habit of suddenly going kaput without any explaination that I've seem.
If you run into a problem where you can't open it and the error you get is an Application Error complaining about the MMC.exe or the MMC Snap-In, try the following:
Navigate to the folder where your Microsoft SQL Server\80\Tools\Binn\SQL Server Enterprise Manager.MSC is. Rename the SQL Server Enterprise Manager.MSC to EntManager.MSC and see if Enterprise Manager opens up. If it does not, then download the SQL Server Enterprise Manager.MSC from the SQL Server CD to the same directory.
This will solve a corrupted or inaccessable Enterprise Manager problem. If your problem remains unsolve after this, you should look for other errors in SQL or you may consider un-installing and re-installing SQL Server as a last resort.