where is the space bar for my iphone ?

If you are referring to the keyboard, the space bar is on the bottom of the keyboard, in the center. If yours does not appear there, could you possibly provide a screenshot of that for us to see? Otherwise, try resetting your phone. Hold the sleep/wake and home buttons together until you see the Apple logo and then release. The phone should reboot. Check after that.

Depending on context, some keyboards do not have spacebar, e.g. Numbers input.