It sounds like your machine is out of sinc between the needle and bobbin parts. Does your machine thread itself or do you do it? You said you had changed threads so you probably threaded the machine right. Is the bobbin in the bobbin cast turning the right way? Or, is there a tiny piece of a needle tip or a wad of thread stuck deep in the bobbin/stitching housing?
If you have done all that is possible and it still doesn't work, I would say it needs to go to the repairman.
Our sewing machine repairman gives his customers a break. If we pay him $35-45 and he will look through and clean the entire machine. If it needs something fixed with new parts, he always calls first. He adjusts everything for that $35-40 dollars. Then on top of that, if the machine stops working during the entire next 12 months, he fixes it for free.
Hope it doesn't need too much work. Does a neighbor have a machine you can barrow until your's is fixed? Later, Gwyn