What would cause my Sharepoint Web Part to go from working to non- overnight?

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SharePoint2010

I've been working on a Sharepoint 2010 Web Part project that worked fine yesterday, but does not work this morning (with no changes).

When I open the page that houses the Web Part in a browser, it now displays:

"Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type PostTravelWizard.PostTravelWizardWebPart.PostTravelWizardWebPart, PostTravelWizard, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b94745c5a5a3d602 could not be found or it is not registered as safe."

Sometimes in the past when this has happened, these kind of problems were fixed when I rebuilt and deployed the project. Howevr, this time When I opened the project in VS 2010, rebuilt, and attempted to deploy, it tells me:

"Error occurred in deployment step 'Add Solution': This solution contains no resources scoped for a Web application and cannot be deployed to a particular Web application."

This blog post says that I need to open the package's manifest file and edit it by adding dummy data such as:

<Solution xmlns="http://schemas.microsoft.com/sharepoint/"> 
 <Assemblies>   
  <Assembly Location="SharePointProject1.dll" DeploymentTarget="GlobalAssemblyCache">     
   <SafeControls>       
   <SafeControl Assembly="SharePointProject1,Version=1.0.0.0, Culture=neutral, PublicKeyToken=****************" Namespace="SharePointProject1" TypeName="*" />     
   </SafeControls>   
  </Assembly> 
 </Assemblies>
</Solution>

The data I currently have there is:

0) "Preview of Packaged Manifest":

<Solution xmlns="http://schemas.microsoft.com/sharepoint/" SolutionId="5f397aa5-be1f-4eb1-a673-314b6da2c706" SharePointProductVersion="14.0">
  <Assemblies>
    <Assembly Location="PostTravelWizard.dll" DeploymentTarget="GlobalAssemblyCache">
      <SafeControls>
        <SafeControl Assembly="PostTravelWizard, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b94745c5a5a3d602" Namespace="PostTravelWizard.PostTravelWizardWebPart" TypeName="*" />
      </SafeControls>
    </Assembly>
  </Assemblies>
  <TemplateFiles>
    <TemplateFile Location="CONTROLTEMPLATES\PostTravelWizard\PostTravelWizardWebPart\PostTravelWizardWebPartUserControl.ascx" />
  </TemplateFiles>
  <FeatureManifests>
    <FeatureManifest Location="PostTravelWizard_Feature1\Feature.xml" />
  </FeatureManifests>
</Solution>

1) In the "Edit Options" area:

<?xml version="1.0" encoding="utf-8"?>
<Solution xmlns="http://schemas.microsoft.com/sharepoint/">
</Solution>

I don't know just what I should edit there - what I see in "Preview of Packaged Manifest" seems pretty much the same as what he is saying to add (with the obvious difference in project names).

Rhetorically I wonder why Sharepoint is so fragile, but my real, direct question is: Why is this happening, and what must I do to get my Web Part working again?