Have you changed the battery in remote key recently? Dropped it in a puddle? Left it near a strong magnet or speaker?
Try the spare key, if it works the other key has lost its immobiliser coding. Only a Seat dealer would be able to recode it or worse find the chip in the key is damaged or even missing.
Trouble is they'll need the car there too. So unless you have a spare key or can sweet talk them into doing a mobile repair, you'll be needing a recovery truck.
Hope this helps