Other details :
On the server where sharepoint is installed, there is also recently added a
folder called "Public" (not by me).
Could that have something to do with my problem  ?
(This happened last week, and I wasn't there, so I have to search a little.)
Also, I cannot consult my http://servername/public any more .

it is added when u install SPS. its part of SPS. u can create and manage
calendars etc using the link. access it and c the features available. use it..

One other thing : when on the server, I try to open  the Sharepoint Portal
Server Administration , I get this message  : "unable to open the sharepoint
portal server servername"

Make sure the netbios name of the server goes to the default
website, and have you change and settings on there?

Check the ipaddress on the properties page for the website. if it
doesn't say all unassinged click ipaddress. Make sure the netbios
name is listed as an identity.

If it says allasigned, check any other websites and make sure that
the netbios name hasn't been assigned there. For some reason, the
app management pieces seem to get flaky if you put them through
another site, even if the setting seem to be identical otherwise.