Understanding how the latching mechanism works may help you to fix this. I'll explain as best I can.
The latching mechanism basically comprises a microswitch, a solenoid & a spring. The solenoid is actuated electrically either by using the handle (which operates the microswitch) or by pressing the remote fob (which sends the appropriate electrical signal to the solenoid). The solenoid then releases the spring, and it is the force provided by the spring which actually unlocks the door. The spring is reset each time you close the boot.
Pressing the emergency release (the black button) releases the spring directly, bypassing both microswitch & solenoid.
For all this to work, there has to a small gap between the hatchback and the body when the boot is shut, otherwise the force provided by the spring won't be sufficient to unlatch the mechanism. If the hatch is properly adjusted, you can feel this gap if you push (firmly) down on the boot lid - you will feel it give a little. If the boot isn't adjusted correctly, or if there's something trapped between the hatchback and the body, then the boot will jam shut & you'll struggle to open it.
First thing to do is to make sure you're pressing the emergency release correctly - I've just taken a photo of mine & the arrow shows you where and in what direction to push. If that releases the boot, then it's likely an electrical problem & the microswitch is the most likely suspect.
If pressing the black lever doesn't release the boot, the next thing is to try pressing down on the boot lid at the same time as operating the release (either electrically with the handle or mechanically with the emergency release). That might take enough pressure off the latching mechanism to allow the spring to release and unlock the mechanism. If this works, and the cause wasn't something caught in the gap, then you will need to adjust the striker plate on the bodyshell so that the hatch fits just loosely enough to allow the spring to unlatch the mechanism.
