What to do to use your PC again depends.
It could be that you don't yet have SP2 installed (you didn't tell). In that case: read http://support.microsoft.com/kb/320397
Deleting unnecessary files from the root folder might help also. The root folder has a limited capacity.
It could be that any of the issues mentioned in http://www.computerhope.com/issues/ch000465.htm is applicable. You're the only one who can diagnose the problem.
Maybe http://www.tinyempire.com/shortnotes/files/ntldr_missing.htm helps. But I don't know how trustworthy this is.
And, of course, it's possible it's just missing (although it's unclear how that came). From the recovery console see if you can find ntldr and ntdetect.com in the root folder using the command dir c:\nt* /a:h (followed by enter). With me, they are. With you?
In that case, get your Windows XP CD, boot into the recovery console and copy it back as described in the last post in http://www.tinyempire.com/shortnotes/files/ntldr_missing.htm
The last recourse would be a repair or even a clean install of Windows XP, but I don't think that's necessary.
when starting my computer "NTLDR is missing- press ctrl+alt+del to restart" massage shows up. same massages shows up no matter how many times i restart my computer. before windows xp was working just fine.
What can i do to have my windows xp working again