Message erreur site :(

Bonjour,

J'ai crée un site internet, j'y ai déjà fait bon nombre de modifications, ce matin tout allait bien mais ce soir quand j'ai voulu y accéder, ce message d'erreur est apparu... Pouvez vous m'en dire plus ?

Merci d'avance de votre aide!
LOU.

Server Error in '/WA' Application.
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Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
ClassLibrary.Do_Boom.Knooppunt_Zoek_Status(String BoomID, String Knooppunt, Object Request, String WebID) in C:\WA\ClassLibrary\Domein_Tier\Do_Boom.vb:724
WA.Index.ToonBoom() in C:\WA\Presentation_tier\Index.aspx.vb:366
WA.Index.Maak_Pagina() in C:\WA\Presentation_tier\Index.aspx.vb:343
WA.Index.Page_Load(Object sender, EventArgs e) in C:\WA\Presentation_tier\Index.aspx.vb:301
System.Web.UI.Control.OnLoad(EventArgs e) +67
System.Web.UI.Control.LoadRecursive() +35
System.Web.UI.Page.ProcessRequestMain() +772