the octane level of the fuel in your gas tank goes down. the worse that can happen is you rev the engine and it knocks. or there is more diesel than gasoline so the engine does not run at all.

first thing i'd do is to fill up to full tank to dilute that diesel. 2nd, add octane enhancer and stay in low revs (below 3k) until the next full tank.

or drain the tank tutal sabi mo 1/4 level na lang.