your toilet has a blockage .
find your manhole cover outside ( the fancy name is inspection chamber) lift the cover off, if this is blocked will need some drain rods to clear it
if the inside of your chamber is clear your problem is between the toilet bowl and the inspection chamber.
you can get a large plunger device to clear the bowl its like the sink version but a lot lot bigger you fill the toilet bowl with water then work the plunger up and down till the blockage hopefully moves
wirecutterman, June 2011
