Hi Niwantha,
There is a big difference between the two phrases.
When someone "is resigned", it means the person accepts the outcome of an event that is usually unpleasant.
Example: Jane is resigned to her fate. / Jane resigned herself to her fate.
When someone "has resigned", it means they have tendered their resignation letter to their employer to indicate an intention to end their employment there.
Example: The CEO from an airline has resigned from the company following public outcry over poor service on board.
Hope this helps.
Cheers,
Lance