Hmmm. Could be literally anything from the app to the client machine' hardware to the network to the database and beyond. Really, there's absolutly no way of telling what's wrong with that little info. It's like calling a car mechanic and saying "My car won't start, what's the problem?"

Start by investigating if there's a commonality between the hangs. Is it always the when the same piece of the app is being run. Is it always the same time of the day?

Does the application hang on all client machines? Only on one?

etc, etc

There's a lot of investigation that you need to do first to pinpoint where the problem is coming from.