second monitor not being recognized.

Hi bbarnes49,

We'll be happy to see how we can help.

You said everything was working fine for several years and suddenly the iMac doesn't see the LG monitor any longer.

Were there any changes, such as a software update?

Have you seen and tried the steps found in If your Mac doesn't recognize an external display - Apple Support?

After giving those steps a try, if you still need help just reach back out to us.

Take care!

First disconnect all wired peripherals (including the external monitor in question) and then restart in Safe Mode then restart normally. Then introduce each peripheral and test each one, then introduce the next and continue until all work.