Force yourself to smile even if you don't feel like it. You will start to feel better. It helps if you can find some funny material to watch on Youtube while you are at it. I suggest that you check out Gabriel Iglesias.
do the Panda Dance
Switch gears in your life! Go out and make new friends .. try new things. Maybe time to make a change and be proactive.
But, we all go through some ups and downs in our lives .. it's normal. Just hang in there .. things will improve if you keep trying.