If some of the characters are missing on the product key card are missing, these are the characters that cannot appear in a Windows Product key, try to work out the missing characters.
Letters: A, E, I, L, N, O, S, U, Z
Numbers: 0, 1
If you are unable to work out the missing characters, the best first option is to contact the retailer you purchased the licence from to ask them to provide you with a replacement key card.
If that is not possible, try to contact Microsoft support directly and provide them with proof of purchase and the other details on the key card, they may be able to provide you with the associated product key.