sonyvaiovgn-nr10e/s?

Check if there is something stuck under it.
Make sure it can move back up after you press it. You may need to remove it to clean it's underside.

If you can't remove the space bar, or there is nothing preventing it from moving normaly, you need to replace the keyboard.

If under waranty, have your dealer do the work.
If not, you probably can get a replacement on e-bay or some other exchange/commerce site.

Replacing the keyboard can be tricky.
Make sure you have plenty of space to work, and a very good illumination. An illuminated magnifier is usualy of great help.
Take some photos as you go along to help you remember how everything should be placed.

Electro
January 2013