You can recover your wallet balance from seed by issuing the following command in Sia-UI terminal:

wallet load seed

The UI will then ask for your wallet password (seed). A restart of siad is required. When unlocking your wallet your password will be the same as your seed, you can change your password to be different from your seed from the wallet after unlocking it.

Seed based file recovery is on the roadmap but currently not yet implemented. Until then take care to see Updating the client for tips about backing up your %AppData% in addition to your seed in order to preserve access to renter and host data.