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Is the bad side flashing on the dash? If not, then it is probably the flasher relay gizmo behind the dash...if the dash flashes correctly, it is probably a crossed wire near the bulb...
Your description is totally inadequite for us to tell what is wrong!!!!!!!
Is the turn signal lamp on the dash not working on one side or are the lamps on the OUTSIDE of the car on one side not flashing? Do the outside lamps on the car flash when you select left or right turn?
If it is the outside lamps on one side that don't flash, you likely have a burned out lamp as the flasher relies on current to cycle, low current, no flashing. If the outside lamps flash but the turn signal lamp on the dash doesn't, then the dash lamp is probably burned out.