Can a case decided 1yr ago under section 406 be reopened? Need justice

The criminal court which has acquitted the accused under Section 406 IPC does not have power to open the case. You could have gone for revision to the sessions court immediately after the acquittal.

As one year has passed now, there are bleak chances for you to succeed, however, as stated by you as the acquittal happened without you being aware about it i.e. ex parte, we have a chance stating unfair trial as the ground on which a reappeal may be made.

But, if your grievance is just to recover your stree-dhan and / or dowry, the proper course of action is to file a Domestic violence case or civil suit, whichever may be more favorable depending upon the facts of the case.

If you have sufficient evidence (photos / videos / witnesses / etc.) of their holding the stree-dhan and / or dowry, you can file a Domestic Violence case or a relevant Civil Suit.