Ouch, I'm guessing the AdminCP has the same error? I don't think there is a tool to rebuild the settings cache outside the Admin CP. You edit the contents of cs_value in the settings entry?
I guess you could enter something temporary to bypass the error, something random like this into cs_value
Then you might be able to access Admin CP > System Tab > Cache Management. Recache Cache... settings row. Edit: I haven't tested this, so take a quick backup now.