Update: There’s a change for Level 25 players since the Valentine’s update – see here:https://tstotips.wordpress.com/2013/02/02/what-happens-at-he-bonus-level-for-level-25-players/

Nothing – the game doesn’t start again at level 1 and you don’t get a message saying “game over”. The game just goes into a bit of a holding pattern until level 26 arrives (as it doesn’t come with the Valentine’s update, it may not happen until March).

You can still collect money from houses, restaurants and businesses and send your characters on tasks. You can keep expanding and reorganizing your Springfield. But your XP doesn’t continue to build. This makes sense – XP rewards frequent players by leveling you up (and giving you the free donut or two) a little faster than your your game levels, which gives you access to buildings like Pimento Grove a bit earlier than less dedicated players.

When level 26 is released, you’ll have a 30,000 or so head start on your XP. Why it’s not more and whether or not that’s fair is a question for another time.

Update: Since the Valentine’s update, your XP continues to build to a bonus round, although there’s still no level 26. It seems like EA have been listening!