I think we might need more information than that...
What kind of computer are you attaching it to? What OS is this computer running? How is the external drive formatted? Did it work recently? When was the last time it worked? Did anything change? Does it show up on another computer? How are you connecting it - USB?
If you go to disk management (start, run, compmgmt.msc, disk management), what does it show? Does it show in the device manager (same section, but device manager instead of disk management)?