I just upgraded my macOS to Catalina. Now I cannot access my iPhotos anymore. How can I access them?

iPhoto was discontinued more than 4 years ago. It is no longer developed or sold. It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.

Not wishing to go off topic but I am using Sierra and about to move to Mojave. The iphoto (9.6.1) Library migration to photos library I am trying to do is grinding to a halt. Has stuck at 34% for 12 hrs or more. It is a 500gb iPhoto library !! Will the conversion work better in mojave being a more unto date system ?

The conversion should work, if your iPhoto Library has no issues. The library needs to be on volume, that is compatible with photos. The volume must be correctly formatted (MacOS Extended Journaled, not case sensitive) or on Mojave it could also be APFS. The volume needs to have plenty of free storage for the migrated copy of the library and needs one locally mounted, not a network share.

It will help, if you ensure that the iPhoto Library is working well in iPhoto, before you try to migrate it. remove media, that Photos cannot process, for example older videos with unsupported codecs, scans with unsupported color sync profiles.

If even iPhoto can no longer handle the library, try to repair it with iPhoto.