I have a IBM ps/1 486sx 25 mz system I recently got and it was working just fine with win 3.1 installed on it already. It was fine for three days and the next day I turned it on it said "Missing Operating System"!!!!!!
Someone here said it maybe the cmos battery, changed that, NOTHING!!!
I formatted hard drive and reinstalled, still "Missing Operating System" after restart!!!

What seems to be wrong???
Hard drive, motherboard?
My floppy does not even light up or operate....
HELP??????

Check your cabling if your floppy is not lighting up. Strange that you were able to format and re-install with out the floppy lighting up. Sure it didnt quit on you after you re-installed it? If so maybe your cables got jarred or your bios config for the hard drive got messed up when you took out the battery....just some suggestions

I DONT KNOW IF I CAN HELP BUT WHAT ITS WORTH I KNOW WITH THE FLPPY TRY TO TURN THE RIBBON AROUND CAUSE IF IT NOT ON THE FIRST PIN IT WILL NOT WORK THERE SHOULD BE A RED LINE ON THE CABLE AND THE NUMBER ON THE FLOPPY DRIVE THE HARD DRIVE TRY TO DO THE SAME OR IN CMOS MAKE SURE THE CYLINDER HEADS AND SECTORS ARE RIGHT I HAD THE SAME PROBLEM WITH MY OTHER COMPUTER I BUILT I BUILT THIS ONE TOO IF THE SETTING AINT EXACT IN CMOS THE COMPUTER GIVE MISIING OPERATING SYSTEM ERROR OR HDD ERROR PLUS MAKE SURE THE HARD DRIVE IS GETTING POWER IF YOU HAVE THE POWER BACKWARDS THE SAME THING WILL HAPPEN I HAVE A PENTIUM WITH 36 MEGS OF RAM AND 100 MHZ WITHH FAN 20X CD ROM SAME s--- HAPPENED COME TO FIND OUT SOMEONE HIT THE DELETE KEY AT BOO UP AND ACCIDENTLEY CHANGED THE SETTINGS LET ME KNOW WHAT HAPPENS

You don't have IO.SYS, MS-DOS.SYS and COMMAND.COM on your hard drive. Did you SYS C: after you formatted?
You usually got a different error message if your hard drive failed or if the parameters were wrong in the BIOS for the drive, but these are things to check if the format is right.
Sometimes that error message meant that the hard drive controller had failed or that the ribbon cable was on wrong, but it usually means SYS C:. Try to boot with a floppy and if you cannot, it will probably be the controller.

Thanks everyone for the help, but how can I reboot system when the floppy does not even light up when computer starts. Computer starts up but tops at a certain point before any acknowledgement of booting. I reformatted the hard drive on another system to see if it would boot up in the system it was in. Whats wrong there?

I had the same situation whenever my machine got powered off and the battery was bad. The solution is to go into setup by hitting F1 after you get the error msg. Scroll to the hard drive and enter the correct type number.
To know the correct type number, you must know the number of sectors associated with the hard drive. This can be found by taking the cover off..the number of sectors will be listed on the top of the hard drive. When you are in setup and at the hard drive item, as you enter a type number (1,2,etc) the sectors associated with that type will appear. When the correct number of sectors appear, you can then save the changes and exit setup. The system will reboot after exiting and should find the operating system.
If you need some help just email me or email me and I'll give you my phone number and we can walk thru it if my explantation is not clear. Good Luck