The official Diablo II General forum and Battle.net Technical Support forum have signs that something is going wrong. These threads provide complaints of lost characters. The team hasn’t yet provided a response to what’s causing this, but Blizzard is providing assistance to recover those lost characters. If you are one of the affected players, first go to your Diablo II folder and locate this path: Diablo IISaveEurope
Change Europe to the realm your characters belonged to, and write down the names of each of your lost characters. Then submit your Battle.net account name, realm and name of the characters. With this info, the technical support team will be able to restore your characters.
Or send an email to [email protected] and please include your account name and realm and a list of the affected characters.