My website admin block seems to be missing. I have the login page and as soon as i enter the logon credentials a message states login successfully but my administation menu is missing and i cannot edit my website.
visitors to the website register succesully and get emails confirming this.
Please help as I am desparate. I contacted my ISP to investigate if any changes were done but they are adamant nothing has changed. A previous backed up copy of site behaves the same.
Please help.

Once you've logged in, you should be able to navigate to the admin page manually by opening:

http://www.yoursite.com/admin.php

I suspect that you may have accidentally turned the User Menu block off. Have a look to see if it is visible in system => admin => blocks.

Another possibility is that your webmaster/registered user groups don't have permission to see the User Menu block. You should check that these groups have access rights under system => admin => groups (registered users and webmasters).