Liner Issues
If the crack is just in the liner and there are no structural issues above the liner it's no big deal. If you want to repair the liner you will not be able to just fill in the crack. You will have to grind out the crack, bevel the edges, make a fiberglass repair then fair and repaint. Not a difficult job but one that requires multiple steps and patience and only moderate skill. Should be able to affect the repair for about $100 in materials and your time. The hardest part will be matching the color.