I have a nikon d40 that works great ewxcept for one day i was taking pictures and all of a sudden the camera would not take a picture in auto or manual and theres a warning sign flashing "r11" where the f-stop should be shown..ive reset the camera, changed the battery, lense, all that, and cannot figure out the problem!

Slow down your shooting. The "R11" indicates that you can take 11 more images before the buffer is full and the camera will stop functioning until the images are moved to the memory card.
Select a smaller file size. Nikon cameras allow you to select from several sizes of JPEG as well as a RAW and a RAW + JPEG file. These files vary in size. If you select a smaller file size, the buffer will take longer to fill, allowing you to shoot longerTurn off long exposure noise reduction. This function causes the camera to expose twice for every image, and this can fill up the buffer quickly as those images are processed and written to the card.