I accidentally froze a whole uncut green pepper. I haven't tried anything with it except take it out of freezer. Is this still usable or is it now garbage?
It is certainly still safe to use.
Freezing vegetables will cause the the texture to soften, so it wouldn't provide a very enjoyable crunch, depending on how you plan on using it.
It would be well suited for use in something like a pasta sauce, stew or chili.
3You can also use it for making stock (where you'd throw away the spent vegetables). I keep a bag in my freezer of vegetables that have gotten a bit sad but not spoiled, parsley stocks, etc. to use when making stock.– JoeJan 19, 2016 at 19:31
Yes, you can use it.
As renesis wrote, the texture will change; so it will much better cooked than raw.
Chop it up and use it in a pasta sauce or a soup, sauce or stew.
I'd do a sofrito with it as a base for a lot of things.