You may need service. However, your camera is a computer and every now and then it can get confused and may benefit from a reboot.

Have you tried turning it off, removing the battery for a few moments, then replacing the battery and turning it back on? (Page 299 of the user manual).

Thom Hogan provides a more exacting reboot routine in his Complete Guide to the Nikon D7000:

1. Turn the camera OFF.
2. If you're using battery power, remove the main battery. If you're using AC Power, unplug the adapter.
3. If possible, let the internal clock battery discharge (this normally takes days).
4. With the battery still removed turn the camera ON for several minutes. This step is to ensure any internal capacitors are discharged.
5. Turn the camera OFF.
6. Replace the batteries taken out in Step 2.
7. Turn the camera back On.

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