Not common at all.
Did you happen to drop and reload the GIS data? Did anything else about the GIS data change (other than add/delete/modify features)? Did you move the location of the FGDB that contains the schematic dataset? Did you change any of the sde server information (instance, machine name etc...)? You don't mention a version, so hard to give exact information or suggestions. You can export a schematic dataset from one database to another. Right-click the dataset in catalog and export it. Usually best to keep the dataset in the same database as the base data (SDE in your case).
Schematics keeps track of the classid's and objectid's of the related feature classes and features. So if that information changes, you have to fix things on the schematic side. Schematics also keeps track of database connection information and version information. So if that changes, you have to fix things on the schematic side.
If none of that has changed, then I would suggest trying to just create a new fgdb, new schematic dataset, new diagram template (type depending on your version) and generate a new diagram. If that works, it will isolate a complete software issue or something wrong with that particular schematic dataset.
