1) Dumb question but have you installed any drivers you may need?
2) Do other USB/Firewire devices show up if you plug them in?
3) If it's plugged in via usb, try firewire, or vice versa
4) go to start->run (or windows key+r) and put in compmgmt.msc then go to drive management to see if it's listed there, without a drive letter, if it is, you can right click it and assign a drive letter to it.
5) None of that works, I would suggest speaking to Maxtor tech support.