Lost remote control for hitachi c2126s,

Unfortunately, that's often the case with universal remotes. Sometimes you can get a universal remote to work by trying all the possible codes listed for a brand. Occasionally different codes will work and control different functions. Or you can try another brand of universal replacement.

The only true solution is to purchase a replacement original remote. There are many vendors online selling OEM remotes, but often the prices are ridiculous.

Sad to say, if you can't switch to the A/V inputs from the front panel or from a universal remote, you won't be able to use them. While it's not the best workaround, you can get an RF converter for around $20 (or less) most places, and then you'd be able to use an A/V source on channel 3 or 4.