Linked worksheet disappears

I used to link certain cells in various xls file to pick up data from another xls file. Each of these files were quite large and would become very cumbersome to handle if merged into a single file. The orginating xls file consisted of a number of a worksheets of which only one sheet had cells which had to be linked outside this file. This file itself had some other cells linked to other cells in another sheet in the same file.
Normally it would all work fine. I could keep going to the originating file and update data onto that, and the linked file too did not give me any trouble
But recently at times the originating file would not open the sheet which had the linked files. I used to think that perhaps the sheet got hidden. Then I realised that the problem apparently is due to some issue with Visual Basic. I would have to then go to the other files and remove the linking to this file. Only then would that sheet open
Yesterday I was in a state of panic. My technique simply would not work! Also the file properties showed that the file was last modified more than a year ago, although I had accessed and updated the file less than a month ago
So I did a file search for all xls files modified in the past 2 months. My file now showed up with the correct properties and I opened it to find all the sheets intact
Can anybody explain what is at the bottom of the problem? My PC is an old Celeron running W2K Professional Version 5.0.2195 Service Pack 4 build 2195. I use updated Office 2000, Norton AntiVirus 2004 and ZoneAlarm firewall.