Does the lens have an aperture ring? If so, it may have moved from being locked at the smallest aperture - this will give an error on the D90. Set the aperture ring to the smallest aperture (biggest f number) and use the little slide switch to lock it in place. If the error disappears, that was your problem.
You could also try setting the camera to manual focus, if it will take pictures then it’s probably an issue with the lens.
Also try unmounting and re mounting the lens in case the contacts aren’t well seated.
Failing that, Harrison’s on London road will be able to take a look. Unfortunately, unless it’s something very minor it is unlikely to be economical to repair. A second hand D90 in decent condition can be had for about £100 nowadays, so that might be her best bet.