I just wanted to say that even though it may feel like it, you are NOT alone. You have taken the first step in admitting and recognising that something is not right. Depression can consume a person (I know, trust me I do) and sometimes feels like someone has turned the light off inside you.
Is there anyone that you can confide in? The one thing that I would have loved to have heard from someone when I was at my lowest several years ago is that it absolutely CAN and DOES get better. Based on your description, it doesn't sound like something in particular is wrong (eg, a fight with a spouse) and that in itself can be scary- because you cannot pinpoint or put your finger on exactly what is wrong; and all you know is that something is wrong- that's the horrible and isolating nature of depression/anxiety.
I hope this all made sense.? I'm not good at putting it into words- just want to say that you are not alone. Please talk to someone, your GP is a great place to start.