big christmas-surprise-NTLDR missing on

i was very surprised today-a real christmas surprise!- when i started pentium 4 with w2k-after 36 hours offline- i could not boot up: it says NTLDR is missing, pls push any key. after several weeks of installing and learning the secrets of thbis stable system, this is a big hit in the stomach. there must be a security-hole. but i downloaded all updates. have spybot(recently installed some days ago, have norton firewall etc. have nsystemworks, all updates made.-something suspicious i could see on my cablemodem. i have 3 pcs connected. only with win2k i could see activation of the light even when pc is off and internet in off. also my second pc-pentium 1 has powerdown(automatic shut-off when leaving windows). but there was absolutely no activities of the modem-lights.