Bacteria contamination will release LPS in media. LPS, which is a bacterial toxin will strongly change the cellular metabolism that will lead to a false cell response in you experimental model. So, the best option is to discard the cell.
You need to clean everything in your culture room and to check the media are free of bacteria. When you´re sure all is fine, take a vial of cells from the nitrogen container and put them to grow. You can use 0.2% Normocin™ from Invivogen. When plate is ready, split it to get more plates, so you can prepare more vials to freeze.
Souce: NovoPro 2018-03-01