If you do electrical work in Revit 2017 there is a bug you should be aware of. The result of this bug is that all project Phases (i.e. Existing, New Construction) disappear as seen in the image below.
|All phases missing from Revit project|
Normally, phases cannot be deleted - just combined with an adjacent phase. But, in any case, the last phase cannot be eliminated from a project.
This is really bad and difficult to fix. Just creating new phases is not the answer as the Room/Space elements are technically still in the model but not accessible - remember, those elements are hardwired to the phase they were created in. Even if you create a new phase with the same name, that does not resurrect the Room/Space elements.
Only Autodesk Support can fix this issue. However, on most projects, in the midst of production, the time it would take to send the model and get it back from Autodesk would result in too much downtime for the project and staff. Thus, going back to the last backup, good local file or Windows previous version is the best option.
But you are asking, why or how might this happen? Good question. It appears to be narrowed down to using the Spline Wire command; starting the command, selecting one electrical connector in a family and then canceling the command (Esc key). However, this is not easily reproducible.
Here is what Autodesk currently has to say about this: https://knowledge.autodesk.com/support/revit-products/troubleshooting/caas/sfdcarticles/sfdcarticles/Phases-disappear-from-project.html
Here is a forum post about this same topic: https://forums.autodesk.com/t5/revit-mep-forum/project-phases-disappeared/td-p/6400515
So, if you are using the Wire command in Revit 2017, you should consider checking the Phases dialog before each Sync w/ Central.