Read the error message: it tells you what happened and what to do to start finding out why:
An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
So start with the stack trace which will tell you the file, method, and line that caused the exception.
Then you can look at that code and see what caused that specific exception.
We can't do that for you: we can't run your code with your data - we don't have enough code, or any data - so we can't even tell what the exception message was, much less what line of code it occurred on!
So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. If you don't know how to use it then a quick Google for "Visual Studio debugger" should give you the info you need.
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!