.NET Application Hangs When Calling DLL

Its been running in production for over a year and this is the first time I saw this behaviour. The application is a VB.NET Console Application which is triggered by MQ server. This application uses a class library which is part of the same project(Say 'abc.dll'). In production around 40+ instances of this exe are running and all of them use the some of the functions from 'abc.dll'.

Last week, we noticed that all the 40 instances were shown to be running in the task manager but they were not doing any activity. There was no exception thrown or error logged in Application logs or System logs. To frantically debug the problem, I tried isolating the problem.

With all these 40+ exes hung up, I wrote a small aplication which would use a reference to this 'a.dll', create an object of one of the classes in that dll and execute a method in there. I was printing console.writelines in between to see to which point it gets to before erroring out.

To my surprise, I found that at the point where I was creating an object of a class in 'a.dll' this sample test application would also hang just like the others. No exceptions thrown. So I killed all those applications as a last resort and then when I restarted all of them, everything worked fine.

I couldn't find any explanation for this behaviour. Since it was a critical error affecting a production system(blanking out the application for 3+ hours), I have to give a very good explanation as to why this would occur. how it can be prevented in future.

(Side note: the servers running these appilcation do not have a maintenance schedule and are supposed to be up for couple of months together.