There are many reasons that an Administrator may need to place a site lock. The most common reason seems to be maintenance. In case you need to apply a site lock the following are the steps necessary.

1. Start the SharePoint Portal Server Administrator from the Start menu.

2. Select configure site quotas and locks from the Virtual Server list page

3. Select Manage site collection quotas and locks

4. Enter the name of your site and click View Data

5. Select the type of site lock that you want to apply.

Adding content prevented – Disables file upload capability within WSS and SPS sites.

No Access – Disables access to the entire site.