i had the same problem... was working FINE one day.. then BAM wouldn't work.. so i turned the car off put it in first.. started it up.. and worked fine for 3-4 more days.. then BAM started not working all over again.. had to have the clutch replaced...

mechanic told me after replacing it that the "clutch plate" was completely "colapsed".. to be honest i don't know much about clutches and what not.. so i don't know exactly what that means.. but after the replacement car is running beautifully again.. (just got it back yesterday)