Hello Murat,

Interesting problem, are you connecting it with USB or are you using a USB-to-PS/2 converter (believe it not, it's not uncommon to see people do this)
Have you tried it on another PC? sounds to me like a driver failure than anything.
Did you try reinstalling the drivers of the keyboard via device manager?
Did you try plugging it into a different USB port?

If the problem persists after trying all of this, then replace the keyboard (like what others are saying)