A handful of times I have come across the following error after trying to login to the Joomla backend of a website:
403 - You can not login to the site. Please, contact webmaster
The problem is happening on sites I earlier had upgraded from Joomla 1.5 to Joomla 2.5 using the Jupgrade extension.
To solve this problem:
1) Take a backup of your site (just to be sure)
2) To sort this you simply have to login to your database via phpMyAdmin (In your Cpanel).
3) Here you will have to look for the table called "jos_categories" (You will here have to change "jos" to whatever your prefix is.
4) In _categories you will have to look for the column called "extension"
5) Change the name any rows under "extension" called "com_contact" to "com_contact_details"
You should hopefully now be sorted.