I have been working on a large image in Photoshop for several days and after doing some additional work, my filters were grayed out when they previously worked fine. After perusing numerous solutions on the web I realized that almost all of them suggested changing the Image Mode to RGB and 8-bits and, for most people, this solved their problem. However, I have been working in RGB 8-bit and none of my image elements were anything different. I had a back-up on the image from the day before and the filters appeared and filters worked fine on it. Finally I realized I had increased the size of the canvas. It was about 30,000 pixels by 1000 pixels and when I increased it to over 40,000 pixels by 1000 pixels the problem occured. I solved it by splitting my image into two parts and everything is working fine now.
I realize this won't be a problem for most users but it might be critical for someone in the same situation. apparently Photoshop filters are image-size dependent and there are limits as to how large an image they will work on.