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You may or may not still be on the payment plan, but if all you owe is $1700, you can call(###) ###-####to set it back up. Likely it is still in effect but I would call the IRS to confirm.
The IRS can revoke a payment plan if you don't file future returns, don't pay future balances, or don't meet current terms.
Based on my expierence, the timing of the missed payment and the letter seems very very fast. You should call the IRS to make sure there are not other problems. Again, as long as you owe such a small total, a simple call can reestablish the plan.