The lens release button causes a locking pin to drop into the flange.
With the lens off, press the pin down with a toothpick. If the pin is difficult to press down, the pin itself may be damaged or obstructed.
With the pin still pressed down, press the button. If the button is difficult to press, the button is likely obstructed.
If neither of the above occurs, the pin may not be moving freely because of excess friction. A minuscule amount of oil around the pin may help. (About as much as will lightly coat the tip of the toothpick.)
On some EF adapters, the pin can be accessed after the flange is removed from the front. I have not tried with a camera, but would expect it to be similar. However, it is probably best to take your camera to a service center, as Romeo Ninov advises. Taking it in sooner, while the flange can still be accessed from the front, would likely make it easier and less costly to repair.
If you feel it is not worth the repair costs, another option you might consider is gluing the pin down permanently. Lenses would no longer lock in place, but they should still work normally. The mount should have enough friction that the risk of lenses detaching during normal use should be minimal.