My employer has a company sponsored SIMPLE IRA plan, of which I've contributed the maximum amount ($6,000) for the current tax year. I also have a Roth IRA account established in the previous year. I am single, and don't expect my annual earned income to exceed $60,000. Can I still contribute up to $2,000 to the Roth IRA for this tax year? Is there an income level after which I would not be able to contribute to the Roth IRA?

SIMPLE IRAs and Roth IRAs are not aggregated for contribution limit purposes, one does not affect the other.
The income eligibility phase out for contributory(not conversion) Roth IRAs for single filers is $95,000-$110,000.