USB Ports Don't Work!

I used a USB wireless modem to connect the internet on an office PC. When I plugged it in the USB ports at the back of the PC, it is not recognized and won't work. So I transferred it to the front USB ports where it worked. I later put a pen drive (which had worked before) in the front port and was now not recognized.

All the USB port cannot work again. What am I to do?

Thanks in advance.

Go to Control Panel>System>Hardware>Device Manager (or Windows key + pause/break), and see if there are any yellow splats in there. If there are, right-click and choose Properties, and report back with the error message(s).