I havent used it for a long time Whiskas,but I found this from an old thread :
Incredimail stores the info in the following directory
Incredimail | Data | Identities | {xxx strange number xxx} | Address ..... or Messages
If you find that folder and then tell incredimail to import the address book from there it may sort your problem out
Cant understand why it was ok at first though and messed up after a few days