.. Check the ISO settings-If too low..exposure meter---.
I agree with the ISO reset as a 1st fix. However, make very sure that you made the error, not the camera. The other possibility? Your camera showing the 1st signs of an on-board problem. They only get worse. If it doesn’t fix easily, warm up the warranty and call Nikon. Get a new camera and enjoy grabbing great shots. .
FYI- Reading the manual is also a good idea, but lacks a great deal of needed info. There are many good reference books on the D90 but a stand out is "Mastering the Nikon D90" by Darrell Young. The rookie book? D90 for Dummies.