Road bike: can't shift down on front gears
Either the shifter, cable or derailleur is stuck.
Detach the cable from the derailleur, check the derailleur will move through its full range of motion. Tension the cable by pulling in it with a pair of pliers. Work the shifter and check the cable is payed out and pulled in smoothly. You can also pull the cable housing away from the shifter to expose the cable, grip the cable (gently) there, tension it and work the shifter. That should tell you what is jammed.
Often the cable gets stuck in the cable housing due to dirt, dried up lubrication or corrosion. Replacing the cable and housing is relatively easy and inexpensive.
If the derailleur is jammed you may be able to get it moving by applying a penetrating oil and pushing the cage back and forth to free the mechanism.
If the shifter seems to be the problem things can be a little trickier. The mechanism can be contaminated with dirt or corrosion, but sometimes shifters break and the only recourse is to replace them.