Lines on monitor will not boot up

Lines on the screen are often a bad display or T-CON board within the display assembly. Did you bang or drop your system? If you have you may need a new display.

This series does not offer replaceable GPU board’s, it uses a soldered on GPU chip on the logic board. You can see it here on Step 6 iMac Intel 27" EMC 2639 Teardown

You will need a new logic board or you’ll need to see if someone with the deeper skills can fix your logic board. Here’s the guide Sostituzione scheda madre iMac Intel EMC 2639 27"

Here’s your logic board options:

• 3.2GHz i5 QC, GT 755M 1GB, Apple P/N 661-7516
• 3.4GHz i5 QC, GTX 775M 2GB, Apple P/N 661-7517
• 3.4GHz i5 QC, GTX 780M 4GB, Apple P/N 661-7518
• 3.5GHz i7 QC, GTX 775M 2GB, Apple P/N 661-8123
• 3.5GHz i7 QC, GTX 780M 4GB, Apple P/N 661-8124