@capybruh2345678 Did you make sure to reinstall Roblox afterward? if you don't get the option to reinstall, go to roblox.com/download/client and use the downloaded file to reinstall Roblox. Once you've done that, try to launch a game.
If it still doesn't work, you might have accidentally disallowed Roblox from launching from URLs. I'm not sure how you'd fix this, but there might be some option in the browser to manage default programs that I'm not aware of. If anyone else knows how to do this, let me know.
Right now, your best bet is to reset all browser settings to their defaults, which will remove any custom theme or pinned tabs you have open, but will NOT delete login details, bookmarks, history, or other saved user data. It will delete cookies, though, so you might be logged out of some sites.
If you're sure you want to try this, go to opera://settings/resetProfileSettings and make sure to uncheck "Help improve Opera" (that option sends your browsing data to Opera), then click on "Reset".
tl;dr: reset your browser settings; go to opera://settings/resetProfileSettings and click "Reset" to reset browser settings.
If, by some chance, none of this works, you might want to use a different browser altogether (Firefox is a good choice).