Hey There!

On a W124, the first place i would check is your OVP relay or "Overage Voltage Protection" relay. It is a common failure and when it goes, it will do just what your saying, prevent an ignition start. The relay is located behind a black panel behind the right-rear corner of your battery. The relay will have a clear plastic panel on top that you will be able to see a fuse plugged in under it. You have to pull out the whole relay. It is a 4 or 5 prong relay. Onced removed, shake it and listen if it rattles. Smack the relay in your palm for 10 seconds or so, then plug it in and start the car- sometimes it works (mind you it is temporary!). If it starts, then you know the OVP is bad. Even if it doesnt work, dont rule out the OVP. Order a new one online, it is only 60 bucks-

Lots of luck!!!!!!!