My 98 E300 would occasionally fail to start. Everything would work, but the starter would not turn. The first couple of times the engine started after letting it rest an hour or two. I checked a number of items, but I figured it out when I took a long rod and a hammer and tapped the starter. Then it started normally. There are a number of things that can cause a no start condition, but mine turned out to simply be a worn out starter solenoid or severely worn brushes. I replaced the starter, and that cured it. One crude test is to see if your lights burn brightly and your horn is loud. If so, you are getting good current flow (at least to some circuits), and the starter may be the problem.