Ancestry.com and associated sites (Rootsweb and Find a Grave) were down for Monday, part of Tuesday and intermittently today due to a DDoS attack. Think of it like a bunch of people all dialing the same phone number at the same time (and keep dialing for a period of time) to make the lines busy, while people with legitimate reasons for calling aren’t able to get through.
There’s a post about it here.
The site is still slow, but should be back to regular service soon.
If you aren’t sure whether a site is down just for you or for everyone, try this site: IsUp.me. or www.downforeveryone.com (those are both the same site).
Type the web address in the box and hit Enter on your keyboard. The site sends a message to the site you are trying to access and tells you if it was successful or not.
At just before 5 p.m. June 18, Ancestry was down (still/again). Rootsweb was down, but Find a Grave was working.