NTLDR Message during start-up.. I have a custom built compter that has worked great for me for a

Question

I have a custom built compter that has worked great for me for a long time. I had just used it the day before and everything was fine but when I tried to start it up it went through normal boot just fine but just befroe it started up windows on the bottom of the boot screen it said "NTLDR is Missing, Press Ctrl Alt Delete to restart". I restarted but I got the same message and my computer wouldn't continue to Windows Xp. What can I do to fix this? Please help!