iMac graphics card issue??

Reset the System Management Controller (SMC). On your 2014 iMac (without the T2 chip) do the following:

You can also reset NVRAM using the these instructions.

If neither of these resolve your screen aberrations, you should set an appointment for service evaluation at your nearest Apple Store, or Apple Certified Service Provider — all subject to pandemic access status and scheduling.

edwarbri Said:

“[...]Everything looks fine on my MacBook but the iMac is still locked on boot screen with green vertical lines. Can someone advise me what I need to do next please?”

———-

Try the Following:

You Could Try a Manual Reinstall:

You would fully delete the partition. From there, install macOS. Then, use a USB Bootable Installer to install the macOS. Once Installed, Backup the Mac. Finally, contact Software Developers asking for new license keys, informing them that your Mac crashed.

——-

In Depth:

Reinstall the Mac OS that was Previously Installed.

I. Create a USB Bootable Installer:

1. Go Here: How to Create a Bootable Installer for macOS - Apple Support
2. Use: Other Mac (Friend’s? Coworkers?) to create this. If not your Mac, then ask for permission from the owner to use it.

II. Erase the Hard Drive:

1. Boot: into Recovery Mode: Hold Down [Command + R] upon boot
2. Select: Disk Utility
3. Erase: the all partitions

III. Boot From USB Bootable Installer:

1. Plug in: the USB Bootable Installer
2. Hold Down: Option upon Boot
3. Proceed: from there as necessary with the Install

IV. Once Installed...

I. Create a Time Machine First...

Create a Time Machine Backup of your Mac, that way you can have something to restore your Mac from, should anything go wrong with the install, and to migrate it all back. Use an External Hard Drive at least 1.5 times the size of the Internal Hard Drive.

V. Reinstall Manually

1. Contact: the Developer for the license
2. Update: the System Software (except for the macOS)
3. Proceed from there as necessary