There are around a million people that have failed to file their 2007 federal tax return. The IRS estimates that they are owed over $1 billion in refunds. To get the refund they must file by tax day this year (April, 17 2012).
The reason that many people don't file is because they did not earn enough to require them to file. However, they are due a refund based on the taxes that were deducted from their wages. They might also be due an earned income tax credit. This applies to filers with less than $38,646 of income for families with two or more children. The thresholds decrease without children. The law allows a three year time limit to apply for the refund. If they fail to do so then the monies become the property of the US Treasury.
If you have not filed than the Accounting Aisle can get you in touch with a good accountant in your area. They will go over your entire situation and determine if you still need to file.