Cant change from av to any input - Sanyo DP42746 42 in. Plasma Television

First try a simple "reset":

1) Turn TV off. Unplug all TV inputs, and the AC plug from wall. Wait 5 minutes (I mean it, a full 5 minutes!).
2) Connect your normal TV source (cable or satellite) ONLY, then plug in and turn on the TV.
3) You may need to press the RESET key twice after connecting the signal and turning on the DTV to restore some functionality.

Now plug in ONE A/V component at a time to test. You can also try replacing any A/V cables (RCA red/yellow/white, HDMI, etc.), as cables do occasionally go bad.

Make sure TV is "tuned" to channel 3 before pressing the "input" button on remote control This is easiest done with the actual channel up/down buttons on the front of the television itself, or possibly by pressing 03 (or 03, Enter) on the remote control.

You may find that only one A/V input is bad (ex, VIDEO 1 port), while others work.. or they may all be bad.

If other functions of remote control are working (but "input" button still does nothing), could simply be a bad remote. Try eBay for a cheap replacement.

If there is not a manual "input" button on the TV itself (I think not for this particular model), and you still can't change AV input, you are probably out of luck. I recommend read included instruction manual carefully, then contact the manufacturer if needed. Instruction manual may be viewed at:
http://www.manualslib.com/manual/599522/Sanyo-Dp42746.html#manual

Do not attempt to disassemble TV. There are no user-serviceable parts inside, and even an unplugged TV can retain enough electricity (in capacitors, etc.) to seriously hurt a non-trained human. This TV is probably ~ 9 years old; parts are available (eBay, etc), but may not be worth fixing considering huge price drops on new TVs available now.