I croped my comp with "Crop Comp to Region of Interest" and accidentally saved the project.
Now how can I get the full screen comp back?
If Ctrl/Cmnd + z will not undo you'll have to go to the comp settings and then reposition everything in the frame.
Backup - Save versions - Turn on AutoSave
In an instance like this, it also works to duplicate the comp and make such a change to the duplicate.
But there's a more fundamental problem: unless you're making something that WILL NOT be projected, shown on youtube or vimeo, played on a DVD or Blu-Ray, shown at the movies or broadcast over the air, it's unwise to trim a comp to the region of interest in the first place. The reason: the odds are overwhemlmingly against you that the dimensions of the work area will be wrong, and it won't look right... if it will play at all. Or render at all, for that matter.
Are you making something that will be used in further AE work? No sweat -- you're good to go.
Almost always, it's better to nest the comp in question in another comp of one of the commonly-used dimensions, then scale and position the nested comp as necessary.