Thick black stripe on iMac Late 2013 after attempting to upgrade SSD

It's an iMac 21Inch, Late 2013.

Running on High Sierra.

Took it to a shop to upgrade form HDD to SSD.

They had to open the Screen in order to do that.

After setting the SSD in it's place a thick black line started showing up from the next startup.

They tried to clean the connectors. The line disappeared intermittently.

But came back eventually.

Any solutions will be highly appreciated.

Thank You