Old Brother Sewing Machine. I just changed the bobbin but now my presser feet don't seem to be working.?

at least my fabric is not advancing. i have no idea what i'm doing! what could be causing this? the tread pulls out of bobblin and needle freely, so it is not snagged. it was working fine before the bobbin ran out of thread. i've probably had the machine since the 70's. It had a tune up 1 year ago.