How to resolve this server error which suddenly has started.

I have only ftp access to the server . The Framework is 2.0.
The web.config data is as follows:
<?xml version="1.0"?>
<configuration>
        <connectionStrings>
            <add name="sSqlConn" connectionString="Data Source=localhost;Initial Catalog=INC123_new1;uid=INC123_gfatm;pwd=admin12!@" providerName="System.Data.SqlClient"/>
      </connectionStrings>
      <system.web>
    <customErrors mode="Off" ></customErrors>
                <compilation debug="true"/></system.web></configuration>


The error is:

Server Error in '/' Application.

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

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.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

Source Error:

[No relevant source lines]

Source File: c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\23987745\f8e58afd\App_Web_rs7xidvs.12.cs    Line: 0

Stack Trace:

[ViewStateException: Invalid viewstate.
      Client IP: 124.253.23.59
      Port: 49924
      User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.6 (KHTML, like Gecko) Chrome/7.0.503.0 Safari/534.6
      ViewState: 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...]

[HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.]
   System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +106
   System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +14
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +237
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +4
   System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
   System.Web.UI.HiddenFieldPageStatePersister.Load() +207
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +105
   System.Web.UI.Page.LoadAllState() +43
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6785
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +242
   System.Web.UI.Page.ProcessRequest() +80
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +49
   ASP.gfatm_testcharts_ptfp2_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\23987745\f8e58afd\App_Web_rs7xidvs.12.cs:0
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

Version Information: Microsoft .NET Framework Version:2.0.50727.5466; ASP.NET Version:2.0.50727.5456