If you back each-way in, for example, a five-runner horse race and there is a non-runner so the event becomes ‘win only’ then all your stake becomes to win. So a £2ew bet would become £4win. This rule only applies to daily racing (ie: not ante-post or futures events).
In ante-post betting markets (also known as 'futures' events) you get the place terms that were in force at the time your bet was struck. The place terms do not change for your bet if there are subsequently non-runners in that event. They will of course have changed for any new bets that are struck.