If the engine is turning over the first thing is to test for spark. Pull a plug wire and put an old spark plug in the wire and ground the plug to the engine while holding it with insulated plyers. If you don't get spark it is in the ignition system.
If you get a good spark you need to check for fuel. You can spray some starting fluid down the air intake on the throttle body and if it starts breifly, then its fuel.
If the engine wont crank over with the starter the battery may be low or the starter may be bad. If the starter kicks and wont turn over with a good battery the engine could be seized. I hope that its not. Hope this helps.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T