Thank you sgunhouse,
Actually, it did erase 12! The download was still there, but the program itself was gone. Go figure. What a mess. On the field on the right, where you can usually read the email, was nothing but Opera's normal background.
I've fixed the problem, though, by finding out how to export email infos to a hard drive, then deinstalling everything that has to do with Opera, reinstalling Opera (this time only the new versions: Opera 31 and Opera Mail, and then importing emails from my hard drive. It actually worked, thank goodness, so I have everything back now.
For whatever reason Opera Mail downloads every new email twice (probably because I have two different email accounts), but other than that I am a happy person again.