What now?

I'm pretty new to investing in shares. I, like many of us, am still holding shares in Big after I didn't recognise the signs till it was too late. My question is, what happens now? What happens to my money? Let's say I put $100 in, now those shares are worth $50. Now the company looks like it's a hairs breadth away from imploding, what happens to my $50 if it does? My understanding is that shareholders own a portion of the company, are we owed restitution?

Cheers