Greetings Linda:
For immediate assistance, please contact a local certified arborist (yellow pages or Illinois Arborist Association Website).
Without more detailed information (i.e., exact species of your oak tree (white oak, pin oak, black oak, etc.); pictures, history of tree health and vigor; recent construction around or near the tree; location of tree in relation to the landscape; soil hydrology, flooding, drought, fertility, pH, etc.), it is difficult if not impossible to diagnose tree health concerns via this venue.
Feel free to send me some pictures and call me for additional assistance. However, I will likely refer you to a local tree care specialist for an on-site assessment.