The 'Play' button is the 'start debugging' button, which can also be invoked by F5.
It invokes the Debugging Command* of the 'Startup Project', which is the one highlighted in the Solution Explorer.
(* Project Properties/Configuration Properties/Debugging/Command)
By default, the debugging command is $(TargetPath), but if you have moved stuff around that may no longer be correct.
Check that the correct project is the 'Startup Project', and then check its debugging configuration as described above.
Answering policy: see profile.