Hello
Nuria - welcome to the forum!
I'm assuming that by your description you are trying to bring up the 'virtual' keyboard in a Safari session and the space bar is not present - correct?
First, force close the Safari app - double press the home button until the lower taskbar opens; press and hold any application until they begin to jiggle; then find Safari and tap the small x in the left upper corner to shut down the program; hit the home button, and finally try to reopen Safari - does this correct your issue?
If not the second option, restart or reset your iPad -
Directions HERE - do a restart first and then check Safari, if the problem persists, try a reset - if still an issue, please post back to this thread - good luck!