I'm using Apache2.0 on Mandrake 10 for two months. Sudenly yesterday I wasn't able anymore to conect to any of my local websites. I start apache as root and get this message:

[root@vladoboss root]# httpd2
httpd2: Could not determine the server's fully qualified domain name, using 192.168.0.1 for ServerName
(98)Address already in use: make_sock: could not bind to address [::]:80
no listening sockets available, shutting down
Unable to open logs

[root@vladoboss root]#

I didn't messing around in any Apache configuration file. It just stop working?! The first part of message is ok, I was getting before, but the second part (Address already in use) is the problem.
Acording to "pstree" Apache is up and running. WTF????

Assuming that you haven't started another process that works on port 80, you may have an Apache process gone weird.

Try doing:

ps -efa | grep httpd

This will show you if there is an Apache process running. If there is, kill it and try re-starting again.

If you don't see an Apache (httpd) process running, try using netstat to find out what's binding to port 80 and we'll try to work from there.

Cheers

Apache is running. When I run as root: nmap -sT -O -p 80 -PI -PT 192.168.0.1 I get this message:

Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-06-02 00:20 CEST
Warning: OS detection will be MUCH less reliable because we did not find at least 1 open and 1 closed TCP port
Interesting ports on vladoboss (192.168.0.1):
PORT STATE SERVICE
80/tcp open http
Device type: general purpose
Running: Linux 2.4.X|2.5.X
OS details: Linux Kernel 2.4.0 - 2.5.20 w/o tcp_timestamps

Nmap run completed -- 1 IP address (1 host up) scanned in 4.982 seconds

So on port 80 there is apache running! I don't know how to use netstat to get some useful informations.

Did you look for an httpd process using ps? I think that you just need to find that process or processes and kill it/them. You could just try:
killall -kill httpd

...and run your diagnostic again. If it's gone away, try starting Apache up again.