Probably all that is needed is a re-adjustment of the front derailleur cable. The cable is pulled by the shifter lever to swing it outwards to the bigger rings, so slightly too much cable between the shifter and derailleur prevents the cage from being swung out enough to get to the biggest ring.
There are many guides to derailleur adjustment out there. My favorite is available on the Park Tool Company YouTube channel (link below), but googling will yield several to choose from.
Basic procedure is:
To tighten the cable, find the front derailleur barrel adjuster, it will be at the shift lever if you have mountain bike shift levers or on the frame somewhere along the cable run if you have a road bike.
Unscrew the barrel to tighten the cable, you may have to go several turns to get the cable tight enough.
- Start with the shifter in the lowest gear / innermost position.
- If the cable is visibly loose, tighten until the slack is just taken out.
- Experiment with shifting and making 1/2 turn adjustments until you can reliably get onto all three front rings and shift back down as well.
If you run out of adjustment in the barrel adjuster you will have to re-clamp the cable on the derailleur.
- Shift to the lowest gear / innermost position.
- Undo the cable clamp on the derailleur until the cable can be pulled through (be careful to note how the cable runs through the clamped),
- Wind the barrel adjuster all the way in then back out a couple of turns
- Put some tension on the cable using some pliers (but don't crush it too much), and re-tighten the clamp.
While you have the cable un-clamped, you may choose to check the inner and outer limit screw settings. See a guide on derailleur adjustment for how to do that.