I can't explain why DHCP stopped working on your iMac, but it might help to reset the connection settings on your Mac using these steps one at a time until the problem is resolved, recognizing that, depending on your problem, not all of these steps will make sense:
1) Use the AirPort/Wi-Fi menu bar item to turn AirPort/Wi-Fi off, then on again.
2) Try to select your Wi-Fi network from the AirPort/Wi-Fi menu.
3) In the Network panel of System Preferences delete the AirPort/Wi-Fi item from the left column, then add it back.
4) In the same panel as (3), define a new location and see if you can make that work.
5) In the same panel, select the AirPort/Wi-Fi connection item, click the "Advanced" button, select the "TCP/IP" tab, then click "Renew DHCP Lease". Note that the fix this step implements may be temporary.
6) In the folder /Library/Preferences ( _not_ <yourhomefolder>/Library/Preferences), move the folder SystemConfiguration onto the Desktop, then restart your computer. See if you can now make your AirPort connection work.
These steps may not help, but they'll only take a couple of minutes to try. Before you try these, if you have any special network settings on your Mac for things like DSL credentials, note all your network settings, because the latter steps will destroy them. If you got all the way to the last step, if things don't work any worse than before, you can trash the saved SystemConfiguration folder.
