Recently I found that for unknown reasons the Excel Solver add-in was not working. Opening the add-ins list from the Devloper tab showed that it had become deactivated (also accessible from File-Options-Add-ins):
Clicking the Solver check box returned a message that the Solver file cold not be accessed. Several similar problems were reported on Stackoverflow and elsewhere, but none of the suggested solutions worked for me, and a lengthy session with Microsoft support, ending up with reinstalling Office, also had no effect.
I finally discovered that when the Excel add-ins are accessed through File-Options-Addins the dialog box has a list of “inactive add-ins”, then a list of “Disabled Application Add-ins”, which listed the Solver add-in. I was able to remove Solver from the list (leaving it empty), after which it could be enabled in the usual way.
I don’t recall when or how Solver became disabled, but for anyone with a similar problem, checking the disabled add-ins list may provide a simple fix.