Car just had an oil change and the check engine light came on car will not go over 40 mph

The engine management system has detected a serious fault and to protect your engine from damage has put it into "limp home mode".

Whatever the actual cause is (possibly a false alarm or a faulty or disconnected sensor) the car needs to go to a garage again for a diagnostic check. Unless you have a good and up to date DIY code reader, then all you can do is to check the oil level (may have leaked or been over or under-filled). Leave the car on a level surface for at least an hour after turning the engine off: withdraw the dipstick, wipe it and reinsert and remove to get an accurate reading. You can also check to see if there are any obvious disconnected wires: perhaps the temperature gauge or oil pressure sensors were accidentally left disconnected after the oil change. Don't assume it's the oil change which caused the fault: use your vehicle maintenance guide to make other routine checks such as the coolant level. Simple checks like these may diagnose or even cure the error without needing a garage.