A/C not working please help

Hi guys

I rencently drove to Montana from San Diego CA my A/C and fan were working fine but when I arrived in MT my A/C stopped working and has not worked since what could be the problem? My truck has 60,0000 miles. Also when I turn the know from A/C to off it still makes the hissing sound. /ubbthreads/images/graemlins/shrug.gif

Thanks,
Andrew