Question Signal issue pc to monitor

Jul 18, 2021
Before I start, I am clueless about gaming pcs. Daughter had a pc built and bought acer monitor. All fine for few months then the monitor was displaying no signal when both switched on. Replaced hdmi cable first, didn't help. Swapped the Acer monitor for my work dell monitor to see if fault was with monitor but same issue. Contacted the company who sent troubleshooting guide involving removing memory cards and swapping them, one in, one out etc. Found that when memory card one was in by itself that the signal worked. Sent results of the testing to company who asked me to return the pc. 3 weeks later it came back, said they'd replaced memory cards, graphics card and motherboard, had extensively tested and all good. Set it up with same Acer monitor, and same issue, no signal. Went back to what I had done in the test and removed mem csrd 2 which had allowed the pc to pass signal to monitor before I had sent it back. This time nothing working. I am convinced that the monitor is not the issue because same problem when I swapped to a Dell monitor. But how do I go back again to the company when they maintain that they replaced all the items mentioned and tested successfully. I am very unsure of what I should do now so any advice would be very much appreciated thanks.
What might have changed in the few months since all was good?

Can you supply the make/model of your parts.
Particularly the acer monitor, the dell monitor and the graphics card.
Some monitors with multiple inputs may not detect a signal and automatically switch to the connected input.
Monitor display controls may be necessary.