Hello, look at the title bar of your document:
1) it says gray/8 or gray/16 at the end.
It is the hint that tells that somehow you switched its mode to grayscale.
Go to Image>Mode and select RGB Color.
2) It says Layer Mask/8 or 16
You have a layer mask targeted and therefore can only select a range of gray colors.
An alternative is to look in your layers panel, and see 1) what layer is targeted 2) if a layer, or a mask is targeted.
Hope this helps!
Maybe at collors tab you have selected GrayScale Slider...