Mountain bike front derailleur won't shift up
First follow the cable run from the shifter to the derailleur and check the cable housing is properly inserted in the shifter and all frame stops. If the housing is not secured the shifter cannot pull the cable to activate the derailleur.
Check the derailleur has not been rotated on the frame. This is easily done by bumping the bike into something. The cage should be parallel to the chainrings
Detach the cable from the derailleur, tension the cable a little by pulling on it with a pair of pliers, work the shifter and make sure it shifts through all gears and pulls in and lets out the cable smoothly.
With the back wheel of the ground, manually pedal and carefully push the derailleur cage out to make sure it will move between the smallest and largest sprockets. If the derailleur range is too limited check the limit adjustments and look for anything jamming the parallelogram.
Reattach the shift cable and follow a guide for derailleur set up and adjustment. Park Tool has a good one.