Some of the DRM systems that went down leaving their clients stranded:
1) Gemstar Rocket eBook
http://web.archive.org/web/200608131...kstore.woa/wa/
2) Embiid Publishing
http://webnews.sff.net/read?cmd=read...id&artnum=2091
3) Amazon eBooks (before Kindle)
https://www.mobileread.com/forums/showthread.php?t=7223
There are more, but these are probably the most well-known examples.