I tried it myself, just now and it seems to be offline for me as well. I am told "Our system is currently busy. Please try again in a few minutes." Here is a screen capture:
I believe Google is doing an update of your links in that tool. When I logged in yesterday, it was up, even though some reported it as down. And I noticed my link count went up about 70,000 links or so. But I have yet to analyze the data and bring it to you. I will.
As of right now, the tool is still down.
Update 8:15am: Now it is back up but shows me zero links.
Update: 8:50am: Also the top anchor text report is down, probably related...