Mac compatible external hard drive not being read by new Macbook

I have purchased a new (gold-yay) macbook... and my external hard drive is not being read by the machine.

I have an iMac for home use and my external hard drive works on there. My hard drive has my entire work life on it and enables me to work at home too. I purchased the Macbook to be able to work on the go too... and more flexibly at home. However I can't access anything at the moment! So my Macbook is an expensive ornament.

Help? Ideas? I have a 2nd external hard drive and it's not reading that either.

I don't really know what else to try?