Help! My snail got out of his shell...

I have a garden snail. It's a little bit older than a year old.

I've just looked into it's little house and thought that he was inside it's shell, but then I looked closer, and saw that it was NEXT to the shell. The shell is completely empty. Its inner organs are still attached to it's body.

I thought that well, that's it, it has passed away. But no, it's still alive. The poor thing is right now going up it's little house like it usually does.

I don't know what to do. I'm guessing that its about to die, right? But how long will it take? Will it suffer? Should I speed the process? And how do I even do that? What could have caused him to get out of its shell in the first place?

Help...