How can I disconnect my profile from a dead OSX Server?

I had a mac mini server with OSX server. The profile I user on my iMac was created on the server, and some groups where on the server, which was also a server for TM backups. For now, the user on the iMac is Admin, Managed, Mobile. The server died recently (in fact, with the different downgrades of the server App by Apple, at the end it was just serving files), but the user on the iMac is still locked to it. For example, I can't change password anymore... What can I do to transform the managed account into a normal independent account on the iMac?