My page had pagerank 3 and now it gives an error when i type the site to see the page rank. I worked at the site 2-3 mounths and then i showed a message like: "Please come back later!Under contruction!!"
Your website was under construction and you have lost your pagerank. Maybe it comes back but i am not sure. Maybe you lost it forever and you should try to improve it again
Unfortunately, you might need to start doing whatever you did to get the page rank in the first place.
In my own experiences, if your could keep building backlinks for your domains, you could usually keep your PR for some extents. I used to have a site with only 1 page of under construction but having a PR of 2. Have a nice day,