Delete the install folder manually. It's should be in "C:\Users\yourusername\AppData\Local\Programs" or "C:\Program Files" or "C:\Program Files (x86)" depending on what type of install you have.
Then, open up regedit and delete the keys for Opera's "Installed Apps" entries. There should be Opera subkeys under "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Uninstall" or "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall" depending on the type of install you have. You can just right-click the "Opera version_goes_here" keys and delete them.
You can also delete the "HKEY_CURRENT_USER\Software\Opera Software" key.
There will be an Opera key under "HKEY_CURRENT_USER\Software\Clients\StartMenuInternet" or "HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet" depending on your top of install that you can delete too.
Then, you can download the Opera installer from https://www.opera.com/download. Scroll down to the section for the Opera you want and choose the 64-bit offline Windows package.
If you still have issues, it could be with your Opera data and or settings etc. You can rename or delete the "Opera Software" folders in "C:\Users\yourusername\AppData\Roaming" and "C:\Users\yourusername\AppData\Local" to start over fresh. Best to just rename them if you have something important you need to try to save later.