Error code E7 stops operating. what should i do?

I just checked your user's manual, and unfortunately it only lists error codes up to E5. There are several things I would check, though. You're saying that it stops working; I would remove the cover over the thread path--it's separate from the front and back, and may be removed by the user for cleaning--and check to see if there are any threads wrapped around something, preventing the machine from working. I would also check that the bobbin winder and buttonhole lever are in their correct positions. When it stops, try to sew manually by bringing the hand wheel towards you (ALWAYS towards you--never away from you.)

Once you have ruled out user error, you might want to send an email to Singer customer service, asking what the code means, because while it might not be in your user's manual, it is probably in the service manual.

If those haven't worked, it's time to buy the service manual. I get mine from Sewing Parts Online; I checked, and it's not listed as in stock, but if you will call or email them (calling is recommended), they can and will order it for you, and their charges are reasonable.