Lift your pressure foot, remove the hoop, do not cut top thread, put a new bobbin in, put your hoop back on. Pull your top thread back up just so it's snug, but not too tight & then if possible, back the machine stitches up about 12 stitches, then start from there. That locks your threads in place.
Only small designs can be stitched in full with a single bobbin. Many require 2 or more bobbins to complete a design.