Spark plug out

i was driving heard a pop sound one of my spark plugs came a loose. got it fix. mechanic told me a piston blew now what could be wrong. i have a 2000 mustang gt

Start with having another mechanic take a look at it for you, just ask him to inspect your pistons, don't say anything else. If he says one of them is blown, then you should get two price quotes and go with the cheaper one. If he doesn't find anything wrong with it, the first guy was trying to scam you.

Please note this is not true for all situations and is not fail-safe, but it provides good odds.