The most common cause is the authentication token expiration. Snowflake's tokens expire after a set period. If that is the case, edit your connection in Looker Studio and re-authenticate with your Snowflake credentials. Consider using service accounts for a permanent fix, creating a dedicated service user in Snowflake, granting the appropriate permissions, then using those credentials in Looker Studio.

Another possibility is that Snowflake is blocking Google's IP ranges, so check your network policies in Snowflake and add Google's IP ranges to your allowlist.

If your queries are complex, they might be timing out, so optimize your queries, consider creating materialized views in Snowflake, or increase the timeout parameter.