Then I discovered that the difference between the troubled page and other pages was some additional CSS. The following method helped:

1. download the troubled page by selecting it and clicking "File –> Export –> File"
2. On your computer using any text editor (I prefer Notepad++) cut the troubled code out (in my case the CSS)
3. On the SharePoint site create the CSS file and paste the CSS code into it.
4. Still on your desktop insert the link link to the CSS file (<link rel="…)
5. Upload the file from desktop (just to be secure make a copy of the file on your server)

Up to this point I didn’t solve the problem, because the troubled (yet validated ok) CSS was still loading and making the SharePoint Designer to hang up. But to bypass the problem I now make the following:

6. Before opening the page in SharePoint Designer I open the CSS file and comment out all the CSS code (/**/)
7. I open the page (yeee, it gets loaded)
8. I uncomment the CSS in the CSS file.

And I can edit the page normally. I hope this method helps at least someone else. Worth a try.

On the Server in SharePoint 3.0 Central Administration select Operations tab and select Alternate Access Mappings under Global Configuration.
 

 
Now change the web application you want to edit public URLs for by using the dropdown on the right hand of the page
 

Next click Edit Public URLs
 

And now enter full URLs (including http:// or https://. You can add up to 5 URLs to which the web application will respond

At the end also add the host headers to the Website in the Internet Information Services manager.

Note that this works on the web application level. If you have site collections with custom URLs you can’t configure multiple URLs for each of these (or if someone know the way, I’d appreciate if you left me a comment).

All you need to add to the page are the
 
?PageView=Shared&ToolPaneView=2
 
parameters.

This opens the Task pane for adding webparts. So now you can add any webpart to the page. Some practical uses are to add Content Editor Webparts to the page or related lists,…

It enables you to automatically convert the downloaded Flash video (.flv) to some of the popular video formats – .avi, .mpg, .mp4, .wmv, etc.

It comes also with a built-in (as an option) torrent downloader.

With SharePoint 3 the story is a bit different. When you create a site with host header through SharePoint Central Administration, that site is bound to the host header you created it with. This might cause problems with intranets (for example internal domain is one and external is other). Below is one solution (that I confess, I still have to try out) that might solve this problem:

Modifying ports and host headers on SharePoint

I also use the following trick: When the site is created, I go to the IIS administration console and add the desired host headers. Usually this will redirect to the original host header. Example: site is created with www.piflar.com, I’ve added also host header piflar.com. So when I visit http://piflar.com it automatically redirects to www.piflar.com.

The problem might occur with intranet sites. Internal domain can be http://companyweb2 and external http://intranet.company.com:444. If you visit the second url and redirects you to companyweb2 naturally you won’t be able to open the site.  

The solution can be that you add the default.aspx after the url. So if you visit http://intranet.company.com:444/default.aspx you won’t get redirected and you can see the SharePoint.

The reason for this is that SharePoint lost the connection to the database.

Reasons may be the following:

• SQL or WMSDE database may not be running

• IIS may be running in IIS 5.0 isolation mode
• The account for connecting to SQL or WMSDE might be gone or with changed password
• update – SQL Server Embedded Edition might not be running (check services) – Thanks, deanoc (see the comments)

The solution to these is on the following link:

http://support.microsoft.com/kb/823287/en-us

But I’ve found another reason that could be. Simple: The SQL is running, credentials are also ok and IIS isn’t in the 5.0 isolation mode. The problem may occur if sharepoint simply can’t find the database. Reasons for this may be many: the database has moved, dns has changed, … In my case was the later. By changing the DNS server in the network we’ve had to add one more record manually to point to the correct one and the sites started to work.

Although many do not recommend this, here’s the way to do it:

In Internet Explorer under the menu Tools click Internet Options. In the open window now select the Security tab.

Next select Local Intranet and click the Sites button.

In the new window click the Advanced button and in the next window type in the URL of your SharePoint site. If the site is not using SSL (https:) remember to clear the “Require server verification…” checkbox. At the end click the Add button and confirm all the windows with OK or Close.

The next time you’ll log on to the SharePoint Site check the box Remember my password and that’ll be the last time you’ll log in to that site manually on this computer.

!!!Important!!!

If you’re accessing sensitive data on SharePoint remember to secure your computer account with a password and protect the unauthorized access to this computer, because anyone can access to this site from this computer now!