Aconteceu comigo o erro:
Caused by: org.hibernate.HibernateException: ‘hibernate.dialect’ must be set when no Connection available
Para resolver apenas coloquei o trecho abaixo dentro da tag persistence-unit do arquivo persistence.xml:
<properties>
<property name=”hibernate.dialect” value=”org.hibernate.dialect.MySQLDialect”/>
<properties>
Isso foi para um banco MySQL, para outros bancos segue a lista:
RDBMS Dialect DB2 org.hibernate.dialect.DB2Dialect DB2 AS/400 org.hibernate.dialect.DB2400Dialect DB2 OS390 org.hibernate.dialect.DB2390Dialect PostgreSQL org.hibernate.dialect.PostgreSQLDialect MySQL org.hibernate.dialect.MySQLDialect MySQL with InnoDB org.hibernate.dialect.MySQLInnoDBDialect MySQL with MyISAM org.hibernate.dialect.MySQLMyISAMDialect Oracle (any version) org.hibernate.dialect.OracleDialect Oracle 9i org.hibernate.dialect.Oracle9iDialect Oracle 10g org.hibernate.dialect.Oracle10gDialect Sybase org.hibernate.dialect.SybaseDialect Sybase Anywhere org.hibernate.dialect.SybaseAnywhereDialect Microsoft SQL Server org.hibernate.dialect.SQLServerDialect SAP DB org.hibernate.dialect.SAPDBDialect Informix org.hibernate.dialect.InformixDialect HypersonicSQL org.hibernate.dialect.HSQLDialect Ingres org.hibernate.dialect.IngresDialect Progress org.hibernate.dialect.ProgressDialect Mckoi SQL org.hibernate.dialect.MckoiDialect Interbase org.hibernate.dialect.InterbaseDialect Pointbase org.hibernate.dialect.PointbaseDialect FrontBase org.hibernate.dialect.FrontbaseDialect Firebird org.hibernate.dialect.FirebirdDialect
Mais detalhes em: http://docs.jboss.org/hibernate/core/3.3/reference/en/html/session-configuration.html