Any money that you earn for working in a different state is taxed by the state that you worked in, even though you are not a resident of that state. It doesn't matter if you live in a state that does not have state income tax. You were supposed to file an Illinois nonresident tax return to report the income that you earned for working in Illinois.
The $1,500 that you now owe probably includes penalties for not filing the Illinois nonresident tax return, and interest from when you should have paid the tax until now.
