Defaulted on my IRS settlement, now what?

I was late with my IRS payment this month, what is going to happen? This year I reached an installment agreement with the IRS for over $1000 a month, but I have been having a hard time paying it every month. This month I was late, and when I went to pay, the system wouldn't let me. Now I received a CP-523 notice, stating they will begin to levy my accounts, ets, and wages! Can I call them and try to negotiate a deal to lower my payments? How much time do I have before they begin this levy process? This cannot happen so close to Christmas. Please help!