Wrong type of card?
Too high a capacity for the reader to handle?
Data on card corrupt?
"it will not read" - what exactly happens - do you get the "dong" noise that says it's been plugged in? - do you get a message?
Is this WinXP or WinME - in which case the reader should just work - or Win98 - in which case you'll need software to make the reader work.
Has the reader worked with other cards?
Has the reader worked with this card on a previous occasion?
Give us a few clues to help with what might be going on.