Visual Studio 2008 C# project now can't find my references -- likely causes?

I'm using VS 2008. In one of our web projects, for some reason, I'm suddenly getting yellow exclamation marks indicating missing references. I'm not sure why.

If I look at the .csproj file in a text editor the references in question are in the project folder structure, tucked in a subfolder, and sure enough that folder and supporting files are there, as they always have been.

Here's a sample of one of the troublesom开发者_Python百科e references, as shown in two places (the only two) in the project file where it's mentioned:

  <ItemGroup>
    <Reference Include="AjaxControlToolkit, Version=3.0.20820.30875, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e, processorArchitecture=MSIL"/>


<Content Include="ThirdParty\AjaxControlToolkit.dll" />

There is a ThirdParty folder in the project, and the appropriate version of AjaxControlToolkit.dll is in there. The project's settings haven't changed, to my knowledge, so I'm at a loss why all these references are supposedly missing. Would anyone know how I might diagnose this?

The easiest/quickest thing to do is remove/re-add the reference.

Would be interested to know what happened, though.

As it turns out, this was due to some instability in my system-- when I closed VS and reopened the project, things worked just fine.