The two-stage Cook County appeal: Assessor first, Board of Review second
Two separate filings, two separate windows, and losing the first does not end anything.
Cook gives owners two administrative bites before the state level, and they are independent.
Stage one: the Assessor
An appeal to the Cook County Assessor's office, filed during your township's window after reassessment notices go out. It is reviewed by the office that produced the value. Many are resolved here.
Stage two: the Board of Review
A separate, independent body with its own filing window, which opens after the Assessor's certification for that township. You may file with the Board of Review whether or not you filed with the Assessor, and whether or not you won.
Why file both
They are decided by different people on different records. A denial at the Assessor is not precedent at the Board, and the Board frequently grants relief where the Assessor did not.
After the Board
The Property Tax Appeal Board, or judicial review in the circuit court. For most residential owners PTAB is the practical route: free to file, slow, and it hears the case fresh.
The discipline
Each stage has its own deadline and each is short. Track your township specifically — Cook's windows are not county-wide.