Welcome to the forum! What is the brand and model number of your receiver? You can often download the manuals from the manufacturer websites.
One very common thing to have happen is that a setting on the receiver that tells it which input to look at for a given source (e.g. Apple TV, blu-ray player) gets changed. On some receivers, it's called "Signal Select." The options are along the lines of "Auto," "HDMI," "Digital," and "Analog." The reason that setting exists is because some receivers let you assign input jacks (on the back or front of the unit) to a given source. Check to see if your receiver has that option and if it's been set to "Analog" (or, really, anything beside Auto or HDMI).
