If your income is greater than the standard deductible, the amount over that is taxable income.
If you did not have enough taxes withheld, then you would owe money on the extra. When you say nothing has changed, did you get a raise or work more hours? This could have an affect on it.
The best thing to do is go back and double check all of your entries to be sure there are no typos.
If everything is OK, then you can compare your last years return to this years return line by line to see what is creating the difference.