Where is the smoke coming from? If the cable then it has probably cracked with constant flexing. Cut the broken bit out and remake the connection at the switch.
If the smoke is coming from the motor then it will most likely have burnt out. A new motor could be obtained and fitted but it is usually nearly as cheap to buy a new lawnmower, especially if you can get an end of season bargain at a sale price.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.