Photos locks up regularly. How can I overcome this. Right now after forcing quit it will not reopen.

The most likely reason for a library to malfunction, or fail to open, is damage to the library caused by storing it on an incompatible volume. Using a clean up app/duplicate file app that doesn't work correctly with a photos library, or changing the contents of the library with finder.

Where is your library stored? In your pictures folder, or somewhere else?

Have you ever used any clean up apps, speed up apps, or duplicate file removal apps?

It might be worth trying this initial checks:

1 - Create a new empty library and try with that library open (hold down the option key while starting photos, and click "create new". To switch back to your currently library afterwards, do the option key start again)

2 - Try booting in safe mode - does it still happen?

3 - Try creating a new user account and try when logged into that account.

These checks will tell us if the problem is system wide, or linked to a particular library or user account.

I have not used any cleanup app. Should I?

kenfromthornlie wrote:

I have not used any cleanup app. Should I?

Definitely, no. It could damage the library further. Some cleaning apps are dangerous. See: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Repeated Force Quitting can also damage the library, if you interrupt Photos while it is trying to update the library or during other critical tasks. It can leave the library in an inconsistent state.

Where on your iMac is the Photos Library? In your Pictures folder inside your home folder or somewhere else? How are you using iCloud? Is the library syncing with iCloud Photos, or are you using My Photo Stream?