You will need to apply some Spring lawn fertiliser which is high in Nitrogen and also contains Phosphorous for the roots. You might find though that some of the grass is dead and will need re-seeding? I bought some English Rye grass seed from our local agri-merchant (drought resistant) and when I checked the manufacturer's site it said that the grass only lives 5 years and then needs re-seeding. That's a new one on me because we've had the same lawn in England for 20 years without ever treating it?