Try to work out the missing characters if you can see some of the characters, these are the characters that cannot appear in a Windows product key:
Letters: A, E, I, L, N, O, S, U, Z
Numbers: 0, 1, 5
If you are unable to work out the missing characters, the best option is to contact the retailer to ask them if they can provide you with a replacement product key card, if not, you could try contacting Microsoft Support, provide them with proof of purchase and all details on the product key card, they may be able to provide you with the associated product key, if not, you will need to purchase another Windows licence.