Front derailleur won't change to low gear
Set the shifter to low.

With the shifter at low and the derailleur stuck at high, the cable should feel slack. If it still feels tight, the cable may be jammed or corroded, so replace the cable, or find whatever's crushing the outer cable (if the outer has been crushed you may need to replace it too).

On the body of your front derailleur you should find two limit screws, labelled "H" and "L". If the tip of the "L" screw is making contact, unscrew it (anti-clockwise) to allow the mech to move towards the low position. If the mech doesn't move and you get space at the tip of the screw, one of the mech's pivots is seized.