I tried to update my iMac, but now it's frozen

OK. Reboot holding down the SHIFT key. Does the help?

If not, reboot holding down Command-R to enter recovery mode. Then run Disk Utilities > First Aid to check your internal drive for corruption.

Also: Do you have a Time Machine backup of your entire drive?

jackibwriter wrote:

I tried the first option, holding down the SHIFT key. No luck.

Stopped at 10% again?

I also tried the second option. No luck.

Did it go into recovery try mode or stick at 10% again?

I'm not sure what a Time Machine backup is...?

An external disk drive that you run the built in Time Machine software; it keeps a copy of all your data so if you need to, you can erase the drive and restore all your data.

Reboot (Hold down the power button until it turns off - then press again to restart.

Does the progress bar appear but stick at 70-99%?