When I updated Visual Studio to the latest version, 1 of my test projects stopped running tests and outputted this message:
Test project {} does not reference any .NET NuGet adapter. Test discovery or execution might not work for this project. It is recommended to reference NuGet test adapters in each test project in the solution.
UPDATED: I was using MS Test as opposed to any other test frameworks like Nunit or Xunit.
I had to add the following Nuget packages:
MSTest.TestAdapter MSTest.TestFramework Microsoft.NET.Test.Sdk 
Ok, you can add Nuget packages as asked. But you can also try to disable the following setting (Tools->Options->Test): "For improved performance, only use test adapters in test assembly folder or as specified in runsettings file".
Let me know if it works for you.
If you are using MS Test, try installing
MSTest.TestAdapter via nuget or
if you are using nunit, install
NUnit3TestAdapterlatest versions via nuget.
After installing please restart the visual studio and then you can see the tests running.
Microsoft.NET.Test.Sdk as well as per the accepted answer.
I had the same issue and none of the answers above worked. Installing NUnit3TestAdapter V3.10.0 fixed it.
I had a very similar issue recently with xUnit, the same outcome, however my fix was due to the fact that previously with lower versions of Microsoft.NET.Test.Sdk you didn't need XUnit.Runner.VisualStudio explicitly installed.
When I updated my Microsoft.NET.Test.Sdk to version 15.9.0 it stopped allowing tests to run until I installed the XUnit.Runner.VisualStudio Nuget.
Now, This may seem blatantly obvious, but, previously it would cope without it and still work. Now, it does not. The same will probably be true of other test platforms. It worked for me.
xunit.runner.utility on Rider.My reputation score does not currently allow me to add this as a comment to the accepted answer. For reference, I've appended version numbers to the nuget packages referenced in csharpsql's answer:
MSTest.TestAdapter v1.3.2 MSTest.TestFramework v1.3.2 Microsoft.NET.Test.Sdk v15.9.0 I know that it is stupid, but for me nothing from earlier answers were working. After that I just restart my computer and everything is working correctly :) (My proble was that one day everything was working correctly and next day it stopped working)
For me I felt very foolish after spending hours, trying EVERYTHING, only to find that I had simply forgotten to add the [TestMethod] directive on the unit test method.
I was trying to run an existing project. I had the .NET Core SDK 3.3 installed, but did not have 2.1 installed. This was causing the tests to silently fail.
I ran into this problem too (in VS 2019), which can be found all over the web. I also support @csharpsql's simple solution. I also couldn't directly comment (how annoying Stack Overflow!)
I had used 'Remove unused references' in VS. I did have to restart VS to get the references removed from view. Cleaning and rebuilding posed no problem.
But running all tests from the Test Explorer reported 'Not run' on all tests, without any explanation or error message. Very annoying.
After finding the suggestion here I started experimenting with re-adding the latest versions of those 3 references. That solved my problem. I just remains annoying this unnecessary problem occurs!
It helped to install xunit.runner.visualstudio 2.5.6 in Visual Studio 17.9.
For xUnit install these 3 packages:
1. xunit
2. xunit.runner.visualstudio
3. Microsoft.NET.Test.Sdk
Since its just a checkbox, Ive tried disabling the following setting before anything else, and it worked!
Tools -> Options -> Test -> "For improved performance, only use test adapters in test assembly folder or as specified in runsettings file"
Adding NuGet packages doesn't work for me.
Disable the following setting (Tools->Options->Test): "For improved performance, only use test adapters in test assembly folder or as specified in runsettings file" works.
