If one forgot to say Mashiv HaRuach and did not realize until after he completed the second prayer of Shemone Esray (Mechayei HaMaysim):

1) If he normally says Morid HaTal during the summer months, then he should continue with his Shemone Esray. It is assumed he said Morid HaTal and, although he should have said Mashiv HaRuach U’Morid HaGeshem, after the fact, he does not need to repeat the Shemone Esray.

2) If he does not normally say Morid HaTal, he needs to go back to the beginning of Shemone Esray and start again.