You didn't say the key wouldn't turn before. Also, is the key slot vertical or horizontal?
There are a couple of issues to check, like does the key go all the way in? If someone put something in there while you were busy doing something else, or, some crud just got in there over time and has lodged so as to prevent the mechanism from moving enough to unlock, you will likely have to go to a locksmith. If the crud can be washed away with WD-40 or the like, you might try squirting some in there. You might also try a newer key with the features that engage the pins in the lock a little less worn.
If the key slot is vertical the vacuum system for locking should handle unlocking. If the slot is horizontal, it won't. If the slot is horizontal the problem is likely in the trunk lock alone, if the slot is vertical the problem may involve the vacuum actuator as well (assuming the central locking system functions normally except for this location).
Sorry I can't be of more help, but that is about all I can think of, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)