You should be able to rotate the starter shaft by hand. If you can't then it is probably seized mechanically. If you can but it doesn't rotate when connected (correctly) to a battery then it is shot electrically. Check the brushes to see if they are OK and the commutator to make sure it is reasonably clean. If they all check out, it is likely a winding problem.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown