C# application runs in the background as a process now i cant build the solution and cannot even kill the process

Question

I ran into a very weird problem today with my C# application. I downloaded it today (from a trustworthy source my classmate sent me back the program we do together) At first when i tried to build and run the application, the whole visual studio got frozen and the app didnt even appear. After a lot of trying this problem just disappeared all of a sudden. However, now I cannot even build the solution, because I get the "unable to copy file because it is being used by another process" error message. I checked the task manager and the program runs in the background. I cant kill it because I have no access to do that(access denied message), I cant do that with a terminal command either(Because the terminal says this exe file is not running) and what is more even restarting my pc 2 times didnt make it disappear. It is no more in the processes after a restart, but i can still see it in the details view of the task manager. I just have this program running in the background literally forever now? Someone could explain me what even happened and if there is any way to solve it? I could even code (and build) the program for a while!