What income do they say you got? A W2 or 1099Misc or 1099R or what? Did you get a letter from the IRS? Watch out for scams. Seems like you would have heard about 2016 before now.
Assuming the "claim" you made more money is not a scam in and off itself, it "sounds" to me like your identity may have been stolen and used by someone for earning income. If this is the likely case, you may want to seek legal representation from a lawyer or tax attorney in your locale.
It's also possible that you did earn the income and you may have been lead to believe it was "under the table". If that's the case, it's not under the table and you may owe tax on it.
Request a wage and income transcript from the IRS to see what income was reported to the IRS for 2016 and by whom.
The IRS notice should show the source of the money they claim you made.
A very common item is that you may have sold some stocks or mutual funds, but failed to report it on your tax return. The IRS got a copy of the 1099-B showing the total amount of the sale. The IRS will consider the entire amount as income until you show them the calculation (on form 8949 and/or Schedule D) of your capital gain.
If this is the case, you need to prepare an amended return and send it to the IRS office that contacted you.
that notice probably gives a date to respond to it. make sure the IRS gets your response in plenty of time. otherwise, the IRS will take action based on the info it has and you will find it costly to change things if the IRS is wrong.