If you had previously negotiated an installment agreement on 941's with the IRS and then either 1) stopped making payments, 2) filed a 941 form late, or 3) failed to make current 941 deposits, then you may be in default of your agreement as demonstrated by receiving this CP523-default notice. Is this the end of the world? Not even close. This default notice will show you that your business has 90 days to get your act together before the IRS levies you (or the IRS assigns a Revenue Officer to your case).
What do you do in those 90 days? File the 941 forms that are unfiled and stay current with your payroll. In other words, the current month and not the old 941's. When you and your accountant go back to the IRS in the next 90 days, you'll be in a better position and have extra leverage. So use the 90 days to get back into shape, enjoy the lull in back tax payments and stay current on your current month's tax issues.