It is possible to re-join your games. You'll need to ask the host for each game to re-invite you.

Once you're back in, there's a couple ways you could revert back to where you were. On the game details page, a game host has the ability to revert turns. However, this is limited to the last two rounds (two in-game complete turns). Alternatively, the GMR desktop client keeps an archive of all saves submitted in "Documents\My Games\Sid Meier's Civilization V\Saves\Hotseat\GMR Archive." Someone could find a submitted save from the right time and resubmit that as their turn to get the game to an earlier state.

If it is a public game you can rejoin on your own - in a private game you need to ask the Host. It depends on how long you were out , if the other players are willing t revert ...

But you can always just continue - its sometimes funny to clean up after the AI :)

In general I recommend the Vacation mode if you can't play for a while - and for shorter periods you can ask if the other players in the game are ok with stopping the game for a week. So far I have rarely seen someone say no ...

Worst option is to just vanish or go on vacation w/o a word and then just vanish for a few month (happens unfortunately more often then I like) .

Because if the difficulty level is high the AI gets considerable advantage in that time

I've found that the AI don't use certain features, like if you gain a free tech while being played by an AI, the AI will not choose the free tech, at all, so it will still be there for you.
Strange, but very useful.

yes buut what I meant is on higher difficulties the AI gets advantages compared to the human players so the moment the AI takes over it operates with the AI environment and not with the human environment

Yes, I understand, and I didn't disagree.
That is why most people play on Prince in MP. The only difference I know of above Prince is the bonus/malus provided to the AI, so there is not much point really.