Revit has a default of 20 backups it saves for the central file once created. Just open up one of those backups do a "save-as" select project, there should be options at the bottom right of next to the saveas and cancel. Select the options, within the options the is a box that says make this the central file. In the same menu at the top it asks how many backups to save? Might be a good idea to set that as well.
Hopefully your backups aren't to old.