Make sure that the needle is a correct type for the machine, and also that the needle is inserted correctly. If the needle is in backwards, or is not pushed all the way up to the top (a common oversight), it can cause the problem you describe. If this is not the cause of the problem, it may be that someone broke a needle or created a bad thread jam that threw the machine out of time. Re-timing is something that most people leave up to a technician, which means a trip to the repair shop.