Broken references in solution

Assuming ProjectFirst generates the DLLs for ProjectSecond, to narrow down the issue, you can first build the ProjectFirst and then build the ProjectSecond to see whether the behavior correct. If that works, you probably need to check your build order.

In Solution Explorer, select a project or select the solution. On the Project menu, choose Project Build Order to check the build order. On the Dependencies tab, select ProjectSecond from the Project drop-down menu. In the Depends on field, select the check box of ProjectFirst that must build before this project does.

Additionally, check ProjectSecond to see whether it by default to targeting .NET Client Profile. If it is, change it to regular .NET.

We have a solution with hundreds of projects (> 400) and tens of thousands of source files and for some unknown reason a set of updates got applied to the machine right in the middle of a build, killing the build and closing Visual Studio. When VS was reopened some projects did not load (even after close and re-open) so had to sort through > 400 projects and find those that did not load and manually tell VS to reload the missing/not loaded projects. After that things seem back to normal.