USB (Universal Serial Bus) flash drives are storage devices without any moving parts that are about the size of a cigarette lighter. Sometimes you may encounter problems getting your computer to recognize this device.
It may be that the USB port you are connecting the flash drive to is inactive, either because its internal wires are not attached, or because it is an external USB "hub" that is not plugged in. So make sure that the port is active.
In rarer cases, it may be that the port is not operating correctly or is damaged. For a port on the computer case, its internal wires may not be connected to the correct areas of the motherboard. Try another port, if possible.
Flash Drive Damage
The flash drive itself may be faulty or damaged, and sometimes this is not apparent from a physical inspection. Try attaching the drive to a USB port that you know is operational, and try plugging another USB device into that port to verify that the port is working.
It may be that the data on the drive has been corrupted, by physical damage or malicious programming code like a virus. In this case, you can try recovering the data with the "Flash Data Recovery Guide" linked in the Resources section below.
If this flash drive has been formatted for Mac or Linux, its data may not be viewable on a Windows computer. For Linux, try the "Ext2 IFS" software linked below in the Resources section, and for Macs, try "HFS for Windows."