2004 Rancher trx350 bored out to a 400 locked up

I have a 04 rancher that’s bored out to a 400 and the motor is seized up so I took it to a shop and they said the starter was fine and the Solon pod was good so they tried the pull start and it was locked up as well then they tried to push start it and he said that the motor still wouldn’t turn over but the back wheels were turning what could the problem be it was running right before I parked it