Suspected hardware failure at that age.
Max uptime: 8 years, 122 days!
It was still being used (on a dialup). It stopped working due to the dialup company stopping service rather than a hardware failure.
Has been replaced by a cheap ADSL connection and router. Ironically this had only been an option for about 6 months due to the rural location and no DSLAM at the local exchange.
Wonderful OS although I'm ashamed to say I left telnet open to the public internet.
The same can actually be said for Windows NT4 as well which tends to show up unexpectedly sticking things together.
Edit: some other notes that might be of interest to long running UNIX admins: Firstly the log files had eaten up nearly all the disk space (2Gb) so rotate them! Secondly the clock had drifted by about 5 days so use ntp. Thirdly, don't assume that if you leave something that it'll be sensibly secure in a few years so they need to be kept religiously up to date. Fourthly, plan for connectivity modes to change over time and keep them up to date; the company was down for 4 days whilst BT got their arse in gear (not that they cared as they had 3G that worked reasonably well). Fifthly, buy good quality hardware - it does last!