This happens to me a lot. Hopefully your running Windows so i can help you with this. Anyways, do the following.

Click Start, now right click my computer and click properties. Click the 'Hardware' tab on the top. Now click Device Manager.

You'll see the name of your computer on the top of the list, click the + next to it if its not already branched out. Now plug in your PSP, you should see a change somewhere, a branch should open or something. You will see a yellow question mark next to something like "Unknown USB Device" or "PSP Type A(or B)". Right click it, click properties. Now on the top tabs click 'Driver' and go to the very bottom it should be the 4th selection and click uninstall. Wait for this to happen and then unplug your PSP & plug it back in. The software on the PSP will automatically install the drivers for use with USB.

If your problem persists create a new thread thouroughly detailing your problem. Closed.