At a guess you need to set a dialect in your .config file.
You can find more information about this in the NHibernate docs and search for config NHibernate on Google.
E.G.
Code:
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.0">
<session-factory>
<property name="dialect">
NHibernate.Dialect.MsSql2000Dialect
</property>
<property name="connection.provider">
NHibernate.Connection.DriverConnectionProvider
</property>
<property name="connection.connection_string">
<!-- Your connection string goes here! -->
</property>
<property name="connection.isolation">ReadCommitted</property>
</session-factory>
</hibernate-configuration>
If this doesn't help you, you'll need to flesh out your query with a lot more detail.