Open your Disk Utility (in your Applications/Utilities folder)
insert your flash drive.
You should see a line showing the manufacturers info for your flash drive. Click on that line to select it. Click the Repair Disk button, or click on the erase tab, and then click the erase button, which will reinitialize your flash drive. DO NOT! choose any option to write zeroes to the drive - this will ruin your flash drive.
If the manufacturer info does not show up, then your flash drive is probably toast. You could also try the drive in a PC as a test.
Serendipity is a lucky guess !