First you need to determine if its physical damage or filesystem damage. Most flash drives use FAT16 or FAT32 so:
1. Plug the flash drive into a PC and try to access it in My Computer, if you get messages like "USB Device Malfunction" or "Insert Removable Media" you likely have physical damage. If you don't see a drive letter in My Computer, right click on My Computer, goto Manage, click on Disk Management, if you don't see the flash drive there its likely physical damage.
2. If you see the drive letter in My Computer right click on the drive and choose properties / tools. Under error checking click Check Now & Start. That should fix any file system errors.
2. While the flash drive is removed, On your Mac, go into Applications / Utilities / Terminal and type dmesg. Insert the flash drive, wait 5 seconds and type dmesg again, then paste whats changed here. (Mac people feel free to correct me, i'm going off my FreeBSD playbook)
If its physical damage and you care about the data you need to send it to a company that specializes in NAND level data recovery of flash drives, in the USA you have:
aplusperfect.com ~$175 (shameless plug for my company)
eProvided.com ~$450
OnTrack ~$3000