First check battery voltage with a multimeter, should be > 13V.

Then turn car off, but turn headlights on for 30 seconds or so to remove surface charge. Battery should be 12.5-12.7V.

If you are <10V, you likely have a shorted cell. If you are below 12V you have a flat battery.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)