Yesterday, at around 3a.m., I have in my logs an "dnnUser failed to login in db DNN5" error. Since that time, the site displays the UnderConstraction.htm page and did not revert to normal status, until we restarted the IIS. The strange things here are two:
1. On the same server, there is another DNN site, on a different database and path, that did not encounter any problems.
2. From the logs it seems that the login-fail, was something than occurred only once, however the site turned to a permanent "under constraction" mode, until we restarted the IIS, 5 hours later. No other action was taken.

How can I prevent this from happening again?

Is there any way to change the UnderConstraction.htm page to something else? We have another server with the same site on a different address, so it would be great if we could send the users there until the problem is solved.

Unfortunately the Event Log has nothing. Last user login was 1' before fail, and then, next one was 1' after the IIS restart. Nothing in between.