I was working in ArcGIS 10.2.1 and was suddendly getting some very odd I/O errors on models that had run before. So I decided to close ArcGIS and Reopen. When I did, the custom toolbox was gone from my ArcToolbox. Other custom toolboxes were there. When I try to Add Toolbox nothing happens. I have a several day old backup of the toolbox, and it is 57 MB. The current one, that is not opening, is 2 KB. Where did it go? can I repair a corrupted .tbx . Has this happened to anyone else? Shold I make my toolbox smaller to avoid this problem again?