One day in a world almost identical to our own a person is granted by pure cosmic coincidence is granted the ability and or curse to transform the world around them into a video game.

The world around this person is much like ours in many ways, such as lacking any sense of tangible magic or being relatively mundane in terms of the technology available to them but this cosmic change will cause these factors to become available to our protagonist at least.

This, however, does not mean some fantastical elements did not already exist such as elves or other fantasy races, it's just that they are just a normal facet of life for the denizens of this reality.

Our protagonist is likely to be just an average joe/jane in out of there head with the only advantage given to them (other than the cosmic one) is their knowledge of video games. The cosmic event is likely to get them out of a rut by granting them a boon.

This could be achieved by granting them a stats sheet and quests they have to complete...

Or they could become a boss monster or NPC in a game which looks suspiciously like there previous world...

Or they could be transported into a fictional setting where the new systems they get to play with have already been in effect for millennia...

Or they could have become an overpowered everyman destined to play out a video game plot love interests and all...

Or literally anything else that the phrase 'turned into a video game' could represent.