A little more information would help.
Is it all cards all the time?
Did anything change about the same time as it stopped reading the cards? ie: did you install any new software.
Have you soft reset the Ax and tried again?
Can you read the cards in a card reader?
One of the most common reasons for this is addressed in this thread.
Solution - Axim not turning on, Disappearing CF/SD Cards and Battery Discharge
Also, what ROM version is your Ax running?
A01, A06 or A12?
If it is A12 then you have the most up to date Dell ROM release, which I believe fixed many issues.
But if you have anything lower than that you might want to consider upgrading.