You may have two separate things going on, here. As to the 'disappearing code,' in the IDE (code window) the Procedure View button may have been accidentally activated. At or near the bottom of the code window, just to the left of the Horizontal Scroll Bars, are two tiny icons, Procedure View and Full Module View. Click on the Full Module View icon and see if your code reappears.
As for splitting the app, having multiple users sharing a single, non-split database, sitting on a network drive, is the sure way to repeated episodes of corruption, as Paul said, as well as speed and timing problems, and all manner of strange, odd and curious behavior, including the problem you're having, now! Multiple users simply have to work off of a split database, with each user having their own copy of the Front End, which contains everything except the data/tables, on their respective hard drives, and a Back End with only the Tables on a shared drive.
Being in forced retirement, I spend 8-10 hours a day here and on other Access forums/newsgroups, and over the past eight years have seen literally dozens and dozens of reports of non-split databases causing these kinds of problems as well as massive data lose! The really insidious thing is that a non-split app can work for extended periods of time before the troubles begin! But once it does, they become chronic, which is to say they occur over and over and over again!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007