Hello and good afternoon.
If the pain is isolated only to your lower back and is not radiating to one or both legs, it is very likely to be back spasm.
Next Steps
Right now, it is very important to rest your back, and you can do that by lying down flat on your back, with one or two pillows under your knees, so that both knees are slightly bent(up to 30°). If the pain is unbearable, you can use a muscle relaxant medication but remember that it is not the definitive solution.
To relieve the spasm, fomentation with a hot water bag may be helpful as well.
If the pain doesn't get better soon, it is better that you get examined by an orthopaedic surgeon to rule out any other possibility as well.
Health Tips
For people doing a desk job, it is extremely important to adopt proper posture practices, you can use any online search engine to search for "ideal desk posture" and you will find a link that helps you.
Sitting upright puts the maximum load on your lower back, so that's a position you should avoid when you have pain in that region; standing in front of a high desk with any one leg supported on a small 6 to 12 inch support is a better position for working.
While sleeping, it helps to keep pillow/s under your knees because that relaxes your back muscles. Always avoid sleeping on your belly.
Take care, and I hope you get well soon.