If you mean the space bar key has fallen off, it's easy to replace keys on the keyboard.
If the key has the metal 'clip' still attached to it, you have to carefully remove the clip from the key.
Attach the clip to the actual keyboard first. Check the spacebar key itself to make sure none of the little grips (where the clip fits in to) are not broken.
If they are not broken, place spacebar key over clip attached to main keyboard and press firmly downwards to reattach the spacebar key. The key itself should click into place quite firmly.
If the grips on the key are broken, the key will not bed properly so you have to get a replacement spacebar assembly from your service centre or parts department and get them to fix it under warranty