Normally a complete re-installation would have wiped out at least all the events and schedules (not to mention any display customizations you did to it - event fields, colours, etc.) from the default calendar so you'll likely have to get them back from a backup. Also the contents of the calendarscript/calendars/default/ directory.
You are likely also missing all user information too which will also need to be restored.
In order to get access to the other calendars, just restoring all the *files* from /calendarscript/ directory from the backup *may* allow access the other calendars again as well as retrieve the user data, etc.
I don't know if the plug-ins (if any are installed) will right themselves or not.
It probably would have been a lot less hassle to just do the upgrade.
JMO
Dan O.
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