First open up the nose cover on the machine. As you raise and lower the presser foot, do you see the thing behind the tension move - that pushes a pin in and out. If it moves it in and out, your best bet is to take the tension apart and put it back together. If you can follow a pattern you can follow the instructions to do this. It isn't really all that hard. Here are some instructions.
http://www.tfsr.org/publications/tec...achine_manual/ - look for instructions that are like your machine. Your owner's manual may have that same info. Here is a 401 repair manual - I think you have to scroll 125 or so pages to find it though.
http://parts.singerco.com/IPsvcManuals/306W25.pdf Make sure all the parts to the tension, including the center pin is there. Oh and when you lift the foot and it does not move that thing in and out it is probably something in the foot lift area of the machine. I would have to look that over real good to figure out how to fix that. That lever does more than just lift the foot on any sewing machine - they just work different from one machine to another. Keep us all posted how it goes.