DLL hell problem are the errors which was caused when multiple applications attempt to share a common library in the multiple applications.

Let's understand it with the help of an example to know it more strongly :

Suppose you have created a dll file for addition of 2 numbers. Now, you have created a console application of Calculator and use that dll file for addition of 2 numbers.

So, that application will work fine, Now, suppose you have created an another application and you have upgrade the dll file as per your need like now addFunction of DLL file can take 3 argument, now you install that application on the same machine and now the dll file which is using previously now overwrite with new function.

This upgrade of application 2 can cause the error in application 1. So, now the user can't able to use the Application 1 on their machine. These problem is very serious issue in the past years. But now it has been solved by the versioning of the application.