How to remove a Removable Disk from my Drive A:?

I use a Removable Disk/Thumb Drive on my computer. It attached itself to Drive A:I cannot gain access to Drive A. When I go to "My Computer" it says there is no available space on Drive A. When I insert a diskette in Drive A is says, drive not accessible the device is not ready. My computer is an old, old Compaq Presario 7222. I tried changing drive letter, but it want allow me to do so. Thank you for your help. It is greatly appreciated.