strange login failure

Let my machine on last night, and was logged into my elgg site. I wake up this morning and turn the monitor on. I'm on my 3 column riverdashboard. Where the river should be is my login box. So I refresh and get sent to the index page.

Now I try to login but it won't accept my details. I login it takes me back to the index and offers the login box again. There is no system error message about failed login.

Now I've tried with at least 4 different accounts on this site to login and all are doing the same thing. This includes both admin accounts.

I'm running a 1.6.1 on a Unix server hosted at
Number of plugins, but all was working fine last night.

Does anyone have any preliminary thoughts as to reason, so that I can find out further what the problem might be?

Thanks in advance