Commented Unassigned: Server Error in '/' Application [27759]

Receiving the below error for my application. Can somebody help me resolve this? __My application was running fine and all of a sudden__ (without any code change from our side) the error is popping up.

__Server Error in '/' Application.

Parser Error

Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.

Parser Error Message: Unknown server tag 'asp:ScriptManager'.

Source Error:


Line 96:
Line 97: <div id="Container"><!-- container -->
Line 98: <asp:ScriptManager runat=server ID="Script1"></asp:ScriptManager>
Line 99: <!--Header-->
Line 100: <uc1:Header ID="Header1" runat="server" />

Source File: /Home.aspx Line: 98

Version Information: Microsoft .NET Framework Version:2.0.50727.4253; ASP.NET Version:2.0.50727.4252__
Comments: Thank you for reporting this problem. This error may be related with the @Register tag. Moreover, it seems that this problem has been already reported [here](http://stackoverflow.com/questions/8104908/ajax-unknown-server-tag-asp-scriptmanager). Please check out your code.   Also, I recommend you upgrade to the newest version of AJAX Control Toolkit - 15.1.2.