I've seen this in a movie and I guessed it meant "every thing is bad", but I just want to know the exact meaning of it. I tried to search in many dictionaries and websites such as Merriam Webster and free dictionary but I couldn't find the answer.
I looked up the word: Hellish
hellish /ˈhɛlɪʃ/ adjective
1. of or resembling hell
2. wicked; cruel
3. (informal) very difficult or unpleasant
"It's been hell" would refer to a period where things felt hellish.
However, at least in the US, it's more often used to describe difficult, but not quiet heinously terrible situations. Sometimes it can be used almost flippantly.
"It's been hell at work the last few months between losing the big contract, the constant layoffs, and working for a completely incompetent boss."
It can be used in an exaggerated way such a clerk saying to a fellow clerk, "Where have you been?, it's been hell in here the last two hours short one person during the morning rush"
War is hell, however I believe that, the less serious way the expression is used today, "It's been hell" would tend to understate a soldiers experience. It would also be used too frequently with humor to describe a dealing with the loss of a child or that level of terribleness
The "it's been hell" expression is probably losing it's shock value over time. Someone answering a question about their well being with an answer of "really bad" might be taken at face value and listened to with concern more quickly.