Will my zener diode work again with a polyfuse after a overvoltage?
I think you misunderstand the operation of a Zener diode - for a Zener, "breakdown" does not mean damage - its normal operation is in the reverse breakdown region - there it will attemp to maintain its advertised voltage.  

In your circuit, the Zener will attempt to limit the voltage to 6.8 volts.  If the input voltage rises above 6.8 Volts, the Zener will draw enough current to "trip" the polyfuse.  If the Zener diode cannot handle sufficient current to trip the polyfuse, it may be damaged.