The first 72 hours after a CP2000 or LT11 matter. Here's the playbook.
Don't ignore it, don't panic, and don't call the IRS before you've read the notice line by line. Most notices are resolvable if you respond within the stated window with the right documentation.
Note the notice type in the top right corner, CP2000, CP14, LT11, etc. Each has a specific response process, deadline, and appeal path.