possible causes
time and date of your computer.
old browser?
a certificate is split in to two basically the company has one half and in most cases the web browser has the other half already built in to it. a couple of the big providers did run out earlier this year, they are normally alreay provided but some versions did not have the update already in place.
upgrde ie if possible if you are not using Ie seek "updated root certs" from your browsers profider (opera firef0x etc)
but do check time date year on your local pc and region