You may need a professional cleaning of all the switches internally. If you are up to it. Compressed air with an electronic aerosol cleaning solution is the first approach. Over time these parts develop oxidation. The voltage levels are small and the signal cannot be generated.You need to know what too clean, most are visible. You can also look for internal fuse protection. Make sure when the cover is removed to pull the power first. Some higher end Amplifiers provide protection. You will need a digital multi-meter or equivalent to test the fuses.
So before doing all of that. There is a reset switch. It is located between the speaker switches on the face of the receiver. At the top and in the middle left center of the display. Not at the speaker buttons on the left.
That function may be tripped. Reset it twice and try testing with different speakers if it fails again. It may trip instantaneously if there are abnormal voltage levels present in the circuit that arouse protection.
I have located and offer you the Manual PDF. Go here first and try the following. On page 8 in the manual. Titled, " Name of each Control" (D). There is a location diagram for you to observe. Please take some time and look this over.
Finally, if you need someone to talk too. The You Tube Video has an outlet store tied to it that offers live help.
TEAC AG 790A AM FM Stereo Receiver
Good troubleshooters always take a problem apart one step at time. So go for it. I know you can do it. Fare. Well.