All sentry bots in my game are invisible. They still exist and attack me but I can not see them. Can't use v.a.t.s. on them either.

If you are using mods, take a look here: http://forums.nexusmods.com/index.php?/topic/3629455-invisible-robots-bug/

I have a ~similar~ bug to the one mentioned in that thread.

In my case, there were two Rust Devils who were invisible except for what looked like Synth heads and hands. I could see the head bobbing around and could target it in VATS, but I could not target any other body part. I finally was able to kill it and was even able to loot the invisible body (head remained visible). I thought maybe a mod had installed some clothing that made the body underneath invisible and untargetable but I looted everything and the body remained invisible.

I am using a metric crap-ton of mods, and I have read that the keyword limit may be what is causing these issues; I had just installed a mod that added yet more things to create... going to try to prune down my mod list and see if that helps. It sounds like a lot, but 120 mods isn't much when each little mod does something different. Granted, there are a few ~gigantic~ mods (Project Commonwealth, Homemaker, Alternate Settlements) but there are also small mods that just do one little thing...