ASP.Net/C# How can method string return be stored for later use in another method?

Can someone please advise on how to store the output of this method return in a usable string or char format that could be used as a variable in a different method? I’ve tried passing by value, by reference, in array format, char format implementations, string format implementations, Char.ToString(), String.ToCharArray(), etc.

Why am I getting this error: The ConnectionString property has not been initialized

I have searched and tried everything but can’t figure this one out. I am trying to do something simple but it seems as though I am doing something wrong. Basically, any user that has made a deposit, I want to return true, if they have not, I want to return false. This should be easy I suppose but I am stumped on this.