The information in this article applies to:
The Site Manager has somehow been corrupted or it has inadvertently been password protected or the Site Manager password has been forgotten.
The encrypted Site Manager data file (sm.dat) will need to be replaced with one from a previously saved backup. If no backup exists, then the Site Manager will need to be recreated.
To create a new Site Manager
You now have a new, empty Site Manager.
It is recommended that you backup your Site Manager regularly or whenever you add sites. The easiest way is to click Help then click Backup Personal & Registration data.
Refer also to the article Malware stole my FTP credentials. Password-protecting your Site Manager (with a password that you are likely to remember) and making regular backups is the best way to prevent its being stolen or corrupted.