i Have Fedora 7 Linux , it was working good and suddenly it stps to see any new connected HDD beside the one Linux use .....
what is wrong ??
and what should i do to fix that ???

Disk not formated? Run fdisk -l as root to see whether the disk gets detected at all. If it does, then it needs to be formated. You could use the command line to do so but you could also use something like gparted.

i have already done that and it dont detect any other harddisk beside my own one

It may be hardware related, check to see that all hdd cables between the mother board and hard drive/drives are fully inserted. Some times, due to expansion and contraction from temperature changes, cables can work themselves loose. It is also possible that an ATA hdd cable may be faulty giving the impression there is something connected to the middle or slave connector of the ATA cable. If you do have PATA drive, and there is an optical drive linked to the middle/slave connector of the same cable, try un-plugging it to see if it makes a difference.
Just a couple areas to check.

Also, it is not un-common for bios (setup) settings to be altered during abnormal events such as power outages, Linux OSs also seem to play with bios settings more often than Windows. Check in the bios to make sure settings co-inside with the current configuration.
This is a more tricky area if you are not computer hardware savvy, reading the mother board manual can be helpful for those that are not familiar with this area, most mother board manuals are available at the mother board manufacturer's web site as a free download.

And, a test with a live cd like Knoppix and Gparted is necessary (and easy to do).

thanks for all the replies , but all the things mentioned above could be true only if the HDD is unreadable on the windows OS too ( i have a dual boot ) but this is untrue , the windows os is working with any plugged HDD perfectly unlike the Fedora which don't even fell the HDD. i think i have some kind of a problem in the udev the booting up in Linux take to much time to finish.
waiting for more answers

All fine but did you mount it by editing fstab in the first place?

mount what in the fstab ???!!!!!!!!!!!!!!
any plugged hdd will be mounted manually when it is pluged,no need for fstab editing