Gear cable breaks
While you can still ride the bike with a broken gear cable, it puts you in your smallest chain ring or your smallest (hardest) rear gear (largest/easiest for Shimano Rapid Rise).
To adjust the gear that you are in, but without restoring shifting capability, use the High and Low swing adjustment screws to move the derailleur across. On the front derailleur, you should be able to move the chain into the middle chain ring. On the back derailleur, you may get one or two gears’ worth of adjustment.
If you need a bigger adjustment,
Pull the cable out from the shifter body on the handlebar so that you have the piece with a “bead” on it.
Remove the cable housing from the derailleur, then thread the cable through the derailleur so that the “bead” on the end of the cable sits in the adjuster barrel of the derailleur.
Tighten the cable in place with the derailleur in the gear you want. The adjuster barrel gives you precise control (except on the SRAM X0 and Shimano XTR derailleur, which give these adjusters up for weight savings).
A faster, messier alternative is to stuff a stick into the parallelogram mechanism of the derailleur so that it is held in the correct position for the required gear. However, sticks of the requisite thickness and strength aren’t always available.