After you check in with your doctor or therapist to make sure things are good, the next best thing is to get out of your environment or routine.
Plan a weekend trip, visit another country, if you’re in the city, go to the mountains, and vice versa. When you feel like nothing brings you joy, that’s when you really have to make yourself go out and do new things, even if you don’t feel like it. Maybe sign up for a new class, go to a museum, volunteer at an animal shelter.
Also: make sure you’re getting enough sleep, going outside, seeing friends/family regularly, and eating relatively well! Sleep alone has a huge impact on how I feel.