This may be a security issue. If you have another key try that and also make sure your battery is at at least 11 volts or try jump starting it. You can also try disconnecting the battery for 15 minutes.
Alex
RepairPal Certified shops can diagnose and repair your 1998 Mercedes-Benz SLK230. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.