Just accept that you feel sad, its not wrong to feel sad, sometimes there are times in life where we just do. Don't fight it but if it doesn't lift seek help. You know your own emotional health and whether this is a one off or whether you have had depression issues in the past. Make an appointment with your GP to discuss your sad state of mind if you feel it is creating an issue for you. As you sit and think, think about activities you might like to try? When did you last have laughter? What did you find funny? What things used to make you feel happy?
Think about those things as much as you can. When you are ready try and find someone to go out with - socially, cycling, walking. Make yourself get out of breath by walking as fast as you can for as long as you can. Go to a Gym, Go for a Park Run event - see if you can find someone to go with. Sometimes getting physically exhausted can just reset our mindset and if you can have fun doing it so much the better. Thank you for sharing your post.