I've got to say the process chain user interface really sucks! SAP has to learn on how to improve it's user interface by learning from other software companies! I was through the menu: Attribute -> Display Component to reassign process chain to another component. I created some test chain and need to delete them, but actually my real chain is active by double clicking it, then I click the test chain once and want to delete it (Note here that I only click the test chain cursor once without double clicking it, even if the highlight focus is on the test chain, but actually it's still on my real chain!), then I accidently deleted my real chain. That's a big big chain that I have burned my eyes to create it! There seems no way to restore it?
Here I tell you a lesson that you have to double click chain to really select it and be careful when conduct deletion!