So I updated Ubuntu 13.10 to Ubuntu 14.04 from live USB. I formatted Ubuntu and installed new version. But my Windows 8.1 is gone now, Gparted does not show any Windows partitions..
I am very positive that I've done 'Install Ubuntu alongside Windows'.
Is it possible that this happened, or I for some reason accidentally pressed format Windows and install Ubuntu (very very unlikely)?
Assuming you have a single disk, you should check the partition's size on the disk to see if it take up the whole drive. If so, then you have definitely formatted over your Windows partition. In that case, you should follow the solution for this question found here. If not, perhaps you simply deleted the partition. Or maybe, although unlikely, Gparted isn't seeing it. Run fdisk -l from a terminal just to be sure.
Also, while a long shot, it doesn't hurt to check out boot repair either.
This is a new feature not a bug. https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1265192
Ubuntu developers do not consider descriptions that are not totally clear to be a bug and software was designed to erase drive, so it is not really a bug. JUst that description of process is not clear that, that is what will happen.
Best to try testdisk and see if it can find old NTFS partition. You may be able to recover some data with it. But anything actually overwritten is gone. Testdisk is in the repository you you can download it into the live installer.
http://www.cgsecurity.org/wiki/TestDisk
http://www.cgsecurity.org/wiki/TestDisk_Step_By_Step
If testdisk does not show files, then you can try photorec, but it is a long slow process and you need another large drive to copy data to.
http://www.cgsecurity.org/wiki/PhotoRec
http://www.cgsecurity.org/wiki/PhotoRec_Step_By_Step
Some suggest this for NTFS, but it is not free:
Caution: getdataback is not the same and is a scam.
