I'm a beginner in portrait sketching/painting and I stopped for a long time so WHAT CAN I DO TO RESUME IT?

Try doing copy art, find a picture (a simple picture for starters) that you like and sketch it out. Just do simple drawings until you think you've got it. Painting and sketching are great hobbies, for painting just buy a child's pallet for watercolor, take one of your sketches and darken the lines with a sharpie or ink pen. you can paint one of you sketches or print something off the internet.