Webapi call is throwing an exception

I have hosted an asp.net webapi project in IIS.The api calls to this project throws an error as mentioned below. The same webapi call works fine when the project is hosted into IIS of another webserver.

 [RoutePrefix("api/datacc")]
    public class DataAccessController : ApiController
    {
        [HttpGet]
        [Route("")]
        public int HeartBeat()
        {
            return 1;
        }               
}

Error

*[ReflectionTypeLoadException: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.]
   System.Reflection.RuntimeModule.GetTypes(RuntimeModule module) +0
   System.Reflection.RuntimeModule.GetTypes() +9
   System.Reflection.Assembly.GetTypes() +61
   FluentNHibernate.AssemblyTypeSource.GetTypes() in c:\work\coding\fluentNhibernate\src\FluentNHibernate\AssemblyTypeSource.cs:27
   FluentNHibernate.PersistenceModel.AddMappingsFromSource(ITypeSource source) in c:\work\coding\fluentNhibernate\src\FluentNHibernate\PersistenceModel.cs:80
   FluentNHibernate.Cfg.FluentMappingsContainer.Apply(PersistenceModel model) in c:\work\coding\fluentNhibernate\src\FluentNHibernate\Cfg\FluentMappingsContainer.cs:122
   FluentNHibernate.Cfg.MappingConfiguration.Apply(Configuration cfg) in c:\work\coding\fluentNhibernate\src\FluentNHibernate\Cfg\MappingConfiguration.cs:80
   FluentNHibernate.Cfg.FluentConfiguration.BuildConfiguration() in c:\work\coding\fluentNhibernate\src\FluentNHibernate\Cfg\FluentConfiguration.cs:249
[FluentConfigurationException: An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.
]
   ProjectName2.Data.UnitOfWork..ctor(String databaseConnectionString) in C:\agent\_work\5\s\.....\ProjectName\UnitOfWork.cs:169
ProjectName.WebApiConfig.Register(HttpConfiguration config) in C:\agent\_work\5\s\.....\ProjectName\App_Start\WebApiConfig.cs:66
   System.Web.Http.GlobalConfiguration.Configure(Action`1 configurationCallback) +27
   ProjectName.WebApiApplication.Application_Start() in C:\agent\_work\5\s\....\ProjectName\Global.asax.cs:18
[HttpException (0x80004005): An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.
]
   System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app) +10093921
   System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +118
   System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +173
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +218
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +296
[HttpException (0x80004005): An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.
]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +10075124
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +95
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +254*

I'd bet there is a problem connecting to the database from the new IIS server you are having problems with. This is coming from NHibernate, so it is an issue with the database; however, if it works from one server then it's probably configured correctly. Verify that you can access the database directly from the new IIS server.