I have just made a very important posting over on the Project Support blog at Project Server 2013 and Project Online- What happened to my resources-. This posting covers a recent bug we discovered with Project Server 2013 an Project Online and now have a fix for. The symptom of this issue is that you can lose resources from your plan – and they appear as numbers rather than a name and are converted to material resources. Please take some time to read it if you are using Project Server 2013 or Project Online and have had issues where you saw resources missing from your plans.
*** Update 2/13 - please see Knowledgebase article - http://support.microsoft.com/kb/2933444 for published details on this issue - including a detection script and a SQL command to correct a stored procedure to avoid this problem ***