often the grease in the speedo drives putrifies. on the plastic bodies gauges they can't really be repaired...the metal bodied ones can be taken apart and cleaned but I've never had any luck taking apart the plastic bodies without destroying them.
if your lucky, sometimes if you just run them they will come back to life after a 100 miles or so. but don't count on it.
flasher problems normally work out to be corroded grounds on the lights or incorrect wattage bulbs or just a low battery.
also the mechanical flashers can rust out. the aluminum cover can be removed and sometimes you can chemically derust them and get them working again,