Start with Nikon support or a Nikon service center. Nikon can usually provide a quote before repair, and factory service is often the safest option for accurate diagnosis. In some cases, minor fixes may be inexpensive or even handled at low cost.

A local authorized repair shop is also a reasonable option, especially if you want something more convenient than shipping the camera.

Before sending it in, try the simple check mentioned in the answers: fully charge the battery. A weak battery can sometimes cause error behavior that looks like a more serious fault.

Given the age of the D80, it makes sense to get an estimate first and compare it with the camera’s current value. If Nikon’s quote is too high, you can then decide whether an independent/authorized shop or replacement makes more sense.