There may be something caught in the area around your key that is making it stick. Use compressed air to spray the area around the key to dislodge any particles trapped in there. If this problem is regularly occurring, you can purchase a keyboard cover in stores or online.
http://www.ebay.com/bhp/hp-keyboard-cover
If there isn't anything caught in the keyboard, the spring beneath the key that causes it to pop back up after being pressed may be worn out, or be missing entirely.
[[HP Stream 11-d020nr Troubleshooting]]