As it happened with me some times when I was searching for some problems and found some messages on forums with similar problems, without any solution:

For future readers - I have not found any solution to the problem.
My emails just disappeared, I cannot retrieve them. All passwords, feeds and email account seem to be working without any problems right now.
Just some kind of error, bug or incompatibility.

Too bad Opera has so many of these. And I am saying that as a fan of Opera.

I know this problem. Unfortunately, the only fix I know is reinstall (but it can happen again anytime).
Sadly, we cannot hope for a correction, due the circumstances.

Such a shame.. now everything seems to be working just fine, I mean I'm able to receive and send new emails, but cannot access the old ones. Luckily most of them is present on my email accounts' servers, so I can access them, but digging through hundreds of mails will be problematic...

Are you sure anything happened? If Opera thinks another copy is running, it will simply disable the mail client so that you can't have two programs writing the same file at the same time. And in Linux, the way it "knows" another program is running is by the presence of a lockfile.
Close Opera, then inspect both your profile and mail folders to see if there is a lockfile present. If there is, delete it.

For sure there was no other Opera copy running. As I said, I just casually restarted the system and then BAM, no old emails for me.

What do you mean by "lockfile" ? The file that is named like this somewhere in one of Opera's folders?

Hello,

I've been using Opera 12.10 for past several weeks/months without any major problem.
I also have Ubuntu 12.04.
Previously I have been using Opera on my Windows PC for the past 7-8 years. Totally love the browser.

Today, after a normal restart of my PC, I turned back my Opera browser.
But to my surprise all emails and feeds that I have received and still not read (or in case of e-mails read but saved for later usage, often pinned them down) disappeared or became completely "blank".

I do not really care too much about RSS feeds, as I can always check them once again, but the problem is with my emails, as I had plenty of important data and information there, all of which seem to be gone right now.

What happened?
How can I fix it? Is it even possible?