memory card unaccessable

I have a slot for an sd card on my asus laptop (vista home premium) and until
recently it has been working perfectly. About a week ago it just stopped
working consistently. I insert the card and the comp. makes the little noise
that it usually does, but the remove hardware icon does not appear, the
little automated window that asks what i want to do with it does not appear,
and the card does not show up on 'my computer'. This is an intermittant
problem, if I restart the computer it usually works again, but sometimes not.
Any suggestions on what I can try? I have not changed anything or installed
anything since the problem started.
thanks in advance!