In the last few weeks we’ve had two bikes come in with broken shift cables. Certain Shimano road shifters are prone to eating cables as shown in the picture. Sure a broken cable will stop your shifting. The real issue is if the cable head gets wrapped around the mechanism and buried inside the shifter. Most mechanics can extract it most times, but if not, you may have to replace the shifter. So, change your cables every once in a while, your bike will work better and help you avoid a costly repair.