As a student on an F-1 visa for the first 4 years you are considered a non-resident and need to file a form 1040-NR non-resident tax return. Turbotax does not support non-resident tax returns so if you used turbotax, you did it wrong and you need to file amended returns using form 1040-NR. Starting your fifth year of being in the US, you use the substantial presence test to determine if you are a resident or not for tax purposes. https://www.irs.gov/individuals/international-taxpayers/substantial-presence-test
However, if you were not complying with the terms of your Visa (students on F-1 visas generally can't have "full time work" unless it is through their school) then you would file as a resident or as a dual-status alien. Turbotax doesn't support dual-status either and you may need professional assistance to get your tax filings straightened out.
If you left the US in 2016 but you have US-source income for 2016, then you need to either file a 2016 form 1040 (resident tax return) or 1040-NR (non-resident return) depending on your residency status -- which depends on the kind of work you were doing, the 5 year rule, and the substantial presence test as mentioned above.
If you don't file a tax return, the IRS will keep any money that was withheld from your pay. They will create a substitute tax return in their computer system based on any income reporting documents they have on file, like W-2s and 1099s. The substitute return doesn't give you credit for deductions since the IRS doesn't have to give you any deductions that you don't prove in writing. If the substitute return shows you would get a refund, the IRS won't pay it. If the substitute return shows you owe tax, the IRS may eventually place a lien against any US assets you have (which may be zero of course). If you return to the US and get a job and start filing taxes again, the IRS can confiscate all or part of your refund to satisfy the old debt. There are also penalties and late fees for filing late (or not at all) that would have to be paid if you ever return to the US and start filing taxes again.
