Is a guard sleeping on duty during a night shift surprising to you? As 542458 said, the level of competency that people assume is hilarious. Prison staff are overworked, underpaid, and have miserable jobs.
When was the last time two sets of electronics failed at the exact same time randomly? If there was a voltage spike or something that killed them both how come it didn't affect anything else at the facility? Maybe they were EMP'ed or something? Okay that sounds fantastical, but I hope they do answer questions. There is likely an innocent explanation to all this, but there should be a thorough investigation.
They were probably broken for years.
I'm not saying it's a movie where the moment a camera goes down a 250lb Russian goon comes down and investigates. However cameras not working for years? In the fucking suicide watch cell? You honestly think that's realistic? Honestly those would probably get replaced faster than most cameras...
Maybe there's a few cameras people just forgot were broken and get forgotten about for years but it wouldn't be a typical case if the IT and maintenance team was even half-competent.
Heck, possibly they were dumping their video onto a recorder in the prison guards' personnel room. Right next to the water boiler. With only one socket within easy reach. Go figure.
(The latter example not being as far-fetched as it may seem; our workshop pager system went down several times a day for this very reason until we cajoled an electrician into fitting another outlet...)
What percentage of cameras on BART trains are working at any given time? When was the last time you tested your backups?
As for electronic failure, I would say it's probably common as well. Just search for "Prison conditions in the united states" or "deterioration of prisons". Our prions are overcrowded and under funded. These things are usually reported in regards to the terrible conditions for prisoners (heating constantly failing, mice/bug infestations, leaking roofs, etc), but this also applies to equipment used by guards. We are talking about facilities running old facilities and equipment with minimum maintenance. Even if the camera's didn't fail (I am not up to date on the story, but I assume they both did?), I would be surprised if the quality of the footage was usable (or if it was even recorded).