It could be the clip holding the cable on to the dust cap on the left front wheel has come off.The speedo cable goes through the middle of the front left wheel bearing and the inner cable has a squared off end that is located in the square hole of the dustcap for the wheel bearing.Or the cable has snapped or disconnected from the cluster end or the speedo is faulty.
Check if the clip is fitted first to the inner cable on the dust cap
Then check it's screwed onto cluster properly.
If still no good then unscrew from cluster,jack left front wheel up and get someone to spin it round whilst you check the inner cable is spinning ok at the cluster end,also check that the cluster end cable hasn't rounded off,it should be square.