First thing you can see is if you left some nodes trying to do compositing in the compositor. If you do, maybe you have a brightness contrast node there. You can either delete all your nodes in the compositor or disable “Use Nodes”. The second thing you have to take into account is the color management tab. Maybe you fiddled with some of the setting there. See if things are set to filmic or standard. Remember you can reset settings by hoovering over your mouse and with the delete key or backspace on your keyboard things go back to default.
