I have updated to Catalina, but now cannot access my iPhoto

iPhoto no longer works in Catalina.

You need to use the Photos App instead.

Switch to Photos from iPhoto on Mac - Apple Support

When you open Photos it should automatically migrate your iPhoto library.

I have High Sierra 10.13.1 and have had a same difficulty, but preferring to continue the unsupported iPhoto, even though I did start to get Photos to open the iPhoto LIbrary at one point.

To get iPhotos working again, here is what worked for me:

Open a terminal window (If it's not in your tray, go to Finder, Application, Utilities folder, terminal.app).

This is the command line interface, with unix like commands (like a DOS window in a Microsoft OS)

Type the follow command and press return:

/Applications/iPhoto.app/Contents/MacOS/iPhoto

Voila

(I think the UI of iPhoto is much superior to Photos.)

Would be very interested in knowing if this works for Catalina too.

As per reply from Keith Barkley, the High Sierra solution won't work on Catalina, which won't run 32 bit software. However, there is a work around, though a bit involved.

https://www.pcmag.com/how-to/how-to-run-32-bit-apps-in-macos-catalina

This link gives step by step instructions for installing a virtual machine on your Catalina, that would run Mohave or High Sierra. I don't know if following that scenario, you might still encounter an the problem I experienced, and need to still launch from a terminal. Doubtful that would occur unless because the VM gets a fresh install, but if it does, then same launch by command line would still apply.

Obviously the VM solution is only desirable for those who have Catalina, but prefer the discontinued outdated iPhotos over Photos

This will not work in Catalina. You cannot make Catalina run a 32 bit piece of software.

Whole lot of trouble to run a dead app. There are plenty of alternatives to Photos, if you don't like that.