carefully take a look underneath your car for oil or coolant puddles, don't do anything until you open the hood to check the oil and antifreeze, don't even try to start it, if it's cold take the radiator cap off to make sure there is fluid in it and check the oil level, if empty call them, they'll tow it, fix it, give you a rental car and a couple coupons for free oil changes in the future. They have insurance for this kind of situation, it seems bad but it was only a mistake. even though its a big thing, a mistake is the same as any mistake. He didn't want this to happen and I am sure you'll understand he didn't mean it and it will usually cost the mechanic his job if he tries not to come clean and just admit his error. This stuff happens all the time, you'll get a new engine. if the filter had a double gasket or failed. Or was the wrong size.