We'll need more information to answer this. Did the computer see the phone previously, or has it never worked? If the latter, you may need to install some drivers (if it's a Windows PC). Are you trying to use some sync tool, or just copy media to/from the phone? What phone model is it, what operating system version is it running, and what type of computer (Windows/Linux/Mac)?
If you could post again with a complete description of what you are trying to do and what happens when you connect the phone we should be able to suggest something.
In the absence of any of this, you could just try a different cable or different USB port - it's not unusual for this to be the problem.