If someone ignores summary judgement, and somone later informs them that they think they might have won, can they appeal and try to argue?

I am a lawyer in CT and practice in the area of the law.  Summary judgment is a final judgment for which an appeal may be taken as long as the appeal is timely made.  Usually, you have 20 days from the date of the judgment to file an appeal.  However, if you want to open the judgment to re-argue the merits of the case, most states will permit you to open the judgment if you file a motion to open within 4 months that the judgment entered.  I suggest hiring a lawyer to review the papers from the court and determine whether you may open the judgment and whether it is something you can fight.