If you go to that games "game history" page, on the right side there should be an option to start a game from the shown position. You can move back or ahead through the game from the start to the final position when you resigned, and a new game can be set to start at that point.
When doing this you can also change opponents, select which color to play, select rated or unrated, and change the time controls, so basically you are setting up a new game from a preset position.
As far as I know it is not possible to undo a resignation so it looks like this is what you may want to do. I have never actually tried this feature.
Good luck, and I hope this works for you. Give it a try and come back to this thread and let us know if it accomplished what you want to do!