To do it on the Mac, plug in the flash drive, and open Disk Utility (in your /Applications/Utilities folder). The drive should appear in the list on the left of the DU window.
Select the drive, then click the Erase tab. On that tab, select MS-DOS (FAT) as the volume format, choose a disk name if you want to, then click Erase, and wait.
That said, if your drive just 'didn't work any more' after a while, it may be faulty and it's possible that re-formatting it won't help. (All flash drives do have a finite number of read/write cycles before they don't work reliably any more, too, although it's unlikely you've reached that limit through normal usage.)
Aug 23, 2008 1:22 PM
Format Flash Drive For Mac And Pc Large Files
To begin to format USB for Mac and PC on a Windows 10 computer, insert the USB drive into an open port and then launch the Windows Disk Management Utility. To find it, type 'disk management' or 'format disk' in Windows Search and select 'Create and Format Hard Disk Partitions.' By default, Mac OS X will use the GUID partition table to format the drive. You can use this and still share FAT32 volumes with a PC, but if you'll primarily be using the drive with Windows,. If you want to make your drive compatible with Mac and Windows, you need to format it with a common file system for Mac and Windows. FAT32 and exFAT can be compatible with Mac and Windows and you can choose one after realizing their respective merits and defects based on your demands: FAT32 takes advantages in compatibility.