Why won't My Mazda 3 start it turns over but will not start.

Does t have one on the key. it was taken to a shop to put the AC belt on and I'm thinking they jumped timing so I am putting it back on time and to see if that's what it is

It's likely to have jumped if you've got more than 100 K on the clock. You can check injector pulse and spark if you have both it's most likely the belt died of old age. Mazda Transponder is in the thin black plastic.