most probably because of build targets got changed. edit your csproject file and try replacing these tags
<PropertyGroup>
<VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">10.0</VisualStudioVersion>
<VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
</PropertyGroup>
<Import Project="$(MSBuildToolsPath)\Microsoft.VisualBasic.targets" />
<PropertyGroup>
<PreBuildEvent />
<PostBuildEvent />
</PropertyGroup>
<Import Project="$(VSToolsPath)\WebApplications\Microsoft.WebApplication.targets" Condition="'$(VSToolsPath)' != ''" />
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v10.0\WebApplications\Microsoft.WebApplication.targets" Condition="false" />