There are several reports within Google Webmaster Help forums and on social media that Google Webmaster Tools link reports are coming up blank.
I am personal able to replicate this on a few verified sites I personally have access to.
The link tool report shows "No Data Available" when you load the internal or external link report.
Here is a screen shot:
I assume this is a bug, it has no impact on your rankings, and that Google is looking into it to be fixed.
Do not worry, you are not alone with this issue, many sites are reporting this.
Forum discussion at Google Webmaster Help.
