I have a setup where I have one monitor, main monitor, plugged in via VGA port. The second monitor is plugged in by the HDMI port. I replaced a couple parts a few months ago and it started happening after the part replacement. I replaced my motherboard and CPU, but I had the dual monitors working side by side fine before then. I changed my motherboard from a capacity of 16GB RAM to 32GB, and the CPU I upgraded from quad core 3.0GHz to six core 3.3GHz.
The problem is, when I plug in my second monitor, which is the one in the HDMI port, the screen on it goes black and says "no signal". The screen on the main monitor also goes black, but stays lit. This happens if I plug it in while the system is already up and running or when the system is starting up (Windows 7). When starting up, the problem happens after the Windows logo stays on screen for a bit. The way to fix this is to unplug the second monitor and resetting the computer.
My old specs were (mobo and CPU): Motherboard: M4A88T-M CPU: AMD Athlon II X4 640 3.0Ghz
My current specs are (all components): Motherboard: ASUS M5A97 LE R2.0 AM3+ Motherboard CPU: AMD FD6100WMGUSBX FX-6100 Processor Memory: CORSAIR Vengeance LP 16GB (2 x 8GB) & CORSAIR XMS3 4GB (2 x 2GB) HDD: WD Caviar 500GB Serial ATA HD Graphics Card: MSI Radeon HD 5670 1GB GDDR5 PSU: ULTRA LSP750 750W ATX POWER SUPPLY OS: WINDOWS 7 Ultimate 64bit