Well, Studio does NOT remove anything. So please go to Windows Explorer, select "This PC" and enter *.sdlproj in the search box, then wait until the PC has found your projects. When done, simply note the path and reopen the projects from that path.

This said, I would also add, that managing files and folders is a crucial part of you work. One should never rely on a program doing that for yu, but should create his or her own folder structure and use this. On top of this, a daily backup is a must. In any case - if something happens, regardless of the reasons, you can restore all your work from the backup.

Thank you Jerzy for your quick reply! Of course, I had backuped the important projects and this accident will not cause any big troubles for me. And, of course, I performed search throughout the hard disk - and did nit find anything. Thus, it is not a case of unintentional moving or deletion (because the Recycle Bin does not contain those projects as well). By the way, Projects folder still contain file projects.xml which still lists all the disappeared  projects. I cannot understand by what miraculous coincidence could I delete permanently all the projects from this folder but leave the XML file.

I cannot tell you what happened. But what I certainly know, there is no such bug. Studio does NOT delete files. What could happen, is overwriting in Windows. But Studio will NOT create a project in a folder, which is NOT empty, nor extract a package in a folder which is NOT empty. I am using this program since 2009 and nothing similar has ever happened to me. So even if this is not a satisfactory answer to your problem, it must have been caused by something different than Studio. And if Studio has done that, so I'm afraid this is not the program to blame, but some actions and quick confirmation of warning messages by the user, which caused the trouble. Believe me, sometimes you start something and think, this will run in the background, so you go and do something else. Within this a window pops up and while you were about to click something else, your mouse pointer will be brought to the click position in this newly appeared message from your background task and you confirm it unintentionally... This happened to me quite often... So really, I wouldn't blame the software, but just restore the projects and next time pay more attention to messages shown. And please do not take this as an attempt to criticize you or insult you. It is only the fact, that we are all human and I really have reasons to assume, that it is the user to blame. This of course includes myself too.

Jerzy Czopik said:

But what I certainly know, there is no such bug. Studio does NOT delete files.

Not entirely true...
Studio does delete files when you cancel the project creation wizard - in such case, the folder where you intended to create the project, is completely deleted by Studio. Not only its content, but also the folder itself.

Could be that it's somehow connect with the autogenerated project folder name (that "Project 1", etc.)... I did not do any deep research - that's SDL testers' work - but I have observed such unintentional and unwanted folder deletion multiple times.

Jerzy Czopik said:

But what I certainly know, there is no such bug. Studio does NOT delete files.

Not entirely true...
Studio does delete files when you cancel the project creation wizard - in such case, the folder where you intended to create the project, is completely deleted by Studio. Not only its content, but also the folder itself.

Could be that it's somehow connect with the autogenerated project folder name (that "Project 1", etc.)... I did not do any deep research - that's SDL testers' work - but I have observed such unintentional and unwanted folder deletion multiple times.