There's nothing silly about reaching out for guidance. I would concur with advice already given here. Sit with what you're feeling and see what comes up, and if necessary consult with a counselor or therapist; also, get checked out to see if there's anything physical. It sounds like there are underlying issues that you may need to address.

If you have unresolved grief, from a death or any relationship that has ended, you might be putting off dealing with it, which subconsciously could be bringing you down. Of course, a professional or even a grief group online or in person might help.

You could also try some self help in dealing with the grief, acknowledging that while you can’t do anything about the loss, you can find a way to keep that relationship on-going by incorporating the good things, passions, values, memories, etc., into your daily life. Perpetuate various positive aspects into your own personality, so in a way the relationship lives on within you, rather than externally, where you continue to be embraced by that person from within, a positive feeling.

If you are dealing with unresolved issues from a death or relationship with someone who had a negative affect on you, you could be suffering from PTSD. It may be over, but is still affecting you. You might it helpful to talk with a professional or interact with group of peers who have suffered a similar relationship. It helps to validate your feelings, giving you the right to feel that you were treated unfairly and learn behaviors and habits to get your going back in a positive direction.

There are “self help” books and online groups for grieving all kinds of relationships, on facebook and elsewhere. I find even various memes can give me a mantra for the day. Grief Beyond Belief, Knowing a Narcissist, and others.

Sometimes it's a change in the chemistry of your body. If the depression you feel is out of character for you, it might be something a doctor can address with medication. My advice is to always go to a professional for advice.

One other thing ... when you are feeling down it's not going to help to listen to sad music or watch a sad movie. Make yourself a playlist of warm-and-fuzzy tunes and have a couple of movies on hand that you love that always make you smile

My daughter gets overwhelmed a lot. She's a PhD siudent. I tell her every time to make a list before bed. 3 things that she can accomplish easily by noon the next day. Walk for coffee, walk the dog, check email. Stuff like that. Crossing the things off a list makes one feel they've done something even if something small.
I also tell her that she simply has to take an hour for herself. Something she will enjoy. Maybe a movie or show that she loves, reading a book for fun, crocheting or anything that she can do just for her own selfish pleasure for an hour.

I don't know what's going on with you and I'm not a professional but those two things above will help get you through a bad day and I agree with the others on this list that you should go and see someone. Sometimes it takes someone else on the outside to help you find what is going on on the inside.

A few of my friends have traveled down a similar psychological path. They have found solace by seeking professional help (psychologist). There is NOTHING wrong with finding such help. One of my friends now uses an array or prescription drugs that help him deal with life. His life would be significantly less then good without this help.

Hope you find a positive solution.

Maybe SAD? Maybe too much crap going down? Are you sleeping & eating properly/normally?
If getting out & trying something new doesn't help (adult ed course, card party, volunteering, whatever) seek help, struggling alone hoping for things to improve is just wasting your precious life. You Deserve to feel good!