I am so sad, my 1988 240DL odometer just stopped

Unfortunately, there is no fix for your current odometer. Your car will continue to rack up miles, but there is no way to tell (sometimes you can with a digital odometer). What I can suggest is getting a new.used gauge cluster and track your mileage from there.

*Note..sometimes the pin (s) used on these can go and cause this issue..it can be repairable if you have the time, patience and money...on the downside, if you've driven the car since it broke, no way on getting those miles added on...the upside..a cheaper alternative is replacement (check ebay or car-part.com)