This is why you read about backup before doing this. I usually boot my Linux CD/DVD/USB to look over the HDD and see if it is indeed gone. This is really old advice about just trying it first without installing. No Linux learning involved as I noted at http://tips.oncomputers.info/archives2004/0401/2004-Jan-11.htm
So boot some distro, run it in its Live mode and look around.
If it is all gone. Use the machines factory restore or your last backup.
Please help, I was trying to install ubuntu alongside windows 10 on my custom desktop. During installation I chose the install ubuntu alongside windows 10 option. The next option was to select drive and allocate drive space. I chose the C drive were Windows 10 was installed, and distributed the disk space equally for both windows and ubuntu. Later, I pressed continue, it started to process and it gave me a fail message which I can't remember which one. Later, it was late at night so I decided to try installation next day. Next day I booted my pc and windows was gone from the boot manager. This seemed strange as I never got to commence ubuntu installation, I assumed my boot manager was broken. I tried repair my pc option, I tried command prompts that can fix the boot manager that I found searching online, nothing fixed it. I took my pc to a repair shop and they told me my windows partition was deleted. This seemed so strange, how can I have deleted my windows partition if I never fully installed Ubuntu in the first place? Did I actually deleted it and I have to reinstall it? What can I do? Please help