My Z-axis is limited to 15 mm at height I have a Prusa i3 and I have a problem concerning my Z axis. It stops at 15 mm even the object to print is more than this.

Can you, please, help me to solve this problem.
@Trish is right, it would help greatly to have more details, such as the model of printer, photos of the printer at the upper height limit, and a video with sound of moving the head against the 15&nbsp;mm stop.  

Without those details you might answer your question for yourself (and for SE) by trying these tests and considering the results.  If you have already done these things, it would help us help you if you included your results, and other experimental results, in the question.

The problem could be in the mechanism or in the slicing program.

Using the UI, can you raise the head above 15&nbsp;mm?  On the Prusa3D branded i3m3 the UI will allow you to move the Z axis hard against the top stops.  If the UI can raise it, the problem is more likely to be in the slicing program.

If the UI can not raise the head above 15&nbsp;mm, move has high as you can and listen carefully.  Does it simply fail to move with no noise?  Is there a clicking or banging sound as you try to move higher?

If there is a clicking or banging sound, something hard is interfering.

If it simply stops moving, does the machine reset or just stop with no glitching on the UI?  It is reset, look for a cable problem, such as a short circuit or wires pulling on the controller PCB.  If it is just a silent refusal to move, it could be the Z-axis plug is loose and being tugged by the cable bundle to the head.

If the problem seems to be in the slicing software, check that the Z-height is set properly there.  If good slicing software thinks you have only a 15&nbsp;mm height, it won't try to print higher than that.