Start Opera in safe mode, reset your flags at the URL opera://flags, close Opera and open it normally.
If that doesn't work, you can rename the "Opera Stable" folder in both "C:\Users\yourusername\AppData\Roaming\Opera Software: and "C:\Users\yourusername\AppData\Local\Opera Software" to "Opera Stable old" to start over with fresh settings and data.
You can try uninstalling Opera again, but delete the install folder afterwards before reinstalling. By default it's the "Opera" folder in "C:\Users\yourusername\AppData\Local\Programs".
Note that to see your AppData folder, you'll want to goto "View -> Show" in Windows Explorer (File Explorer) and enable "File extensions" and "Hidden items".