If the phone had problems initially. The you should get back to the retailer straight away.
The hinge should move smoothly, without any force to open it.
Did you purchase it new or used/reconditioned?
It will have a warranty, but if there's any damage that has caused the fault, it may not be covered.
Get in touch with the Samsung Customer Support and book a repair.
They will assess the phone and advise you before any repair is carried out
I hope you get it fixed quickly, as they are a great phone to use