Yesterday Garmin invalidated the passwords of many users of Garmin Connect. If you’re having trouble syncing you’ll need to first login to Garmin Connect. Once there, you’ll be prompted to reset your password. After that go back to Smashrun and update your password on the Settings -> Sync page. You should then be able to sync without issue.
Note: If you’re logged in on Garmin Connect’s site already, you may need to logout and login again to receive their reset your password prompt.