When an 830 is locked up tight like that, the usual culprit is a broken nylon gear. Replacing it is not a job for beginners so you'd need to invest in a good tech near you to do the job. Double check that there aren't any threads would around the bobbin area and hook and use a flashlight to inspect the gears. I had one 830 that was locked up like this and it took me about 3 or 4 times to look it over before I spotted the cracked camstack gear.. If it's locked up the crack may be hidden if the gear is rotated and the crack faces down.