On my Cadillac 2004 CTS on the dash says, Engine Power Reduced ??

Run the fault codes as it may be in limp mode to protect the transmission or engine.

Generally ( as Bill Boyd stated) it's computer control to protect or force you to service a vital part. The most common trigger is in the Throttle body inputs on your model. You can try this if you don't have easy access to a code reader. Just disconnect your battery (+) for about 15 min and then clean the terminal and hook it up again. That resets the computer and you may just luck out. Also make sure the wires leading to the throttle body are on tight and not corroded. If this doesn't help you need to have the fault code read.