First of all: thank you all very much! OrbWeaver: you were right. I have no idea how I thought that the video signal could go to the monitor if it is plugged into the graphics card and not the motherboard. Brainfart on my side. Anyway, after removing it and plugging the monitor into the on-board card, my computer startet up, but could not boot until I rebuilt the Boot Configuration Data. I assume this was due to the motherboard trying to access a GPU that is no longer there.
My current specs are:
Motherboard: ASRock x58 (there is also some Z17Pro4S; not sure if this is some software on it? I am quite certain that x58 is the model and this is something else)
Processor: Intel i5-6600 @ 3.3 GHz
RAM: 2 x Ballistix DDR4 with 8 GB
If I remember correctly, the graphics card is a GeForce GTX 960, but it could also be 860 or something. I don't have the packaging anymore and never bothered to write it down, as I always assumed, I can look it up in the Device Manager. This is, of course, a bit difficult if it is not plugged in...
Now that I am already writing about my PC, I can also include some former events: I have had problems with the PC for some time (hence considering buying a new one). It started to use the system hard drive on full capacity, which considerably slowed down the PC (I had to wait almost a minute for a mouse click to register) until I restarted the PC. This happened the last two times after updating my anti-virus software (first Avast, later switched to Kasperky; both times free version). While running at full capacity it was usually the System writing some files into the anti-virus folders. This is also what happened last Friday. I had to shut it down by cutting the power, because I was visiting friends over the weekend and had to leave. Otherwise I would have let it run over the day and see if it sorts itself out. When I came back yesterday, the PC would not boot anymore. The times before that, the PC would usually not recognise the hard disk anymore (btw a Samsung SSD 850 EVO with 500 GB), so I had to format and install a system image. I first thought it was a problem with the SSD (before the current one it was a Crucial MX200 with 250 GB), especially after the 100% capacity thing and it being an SSD I thought it might be some faulty sector. But after it happened again with the new SSD (which cost me a lot of my data; including any TDM WIPs; mostly due to my own stupidity and carelessness), I started to look for other causes. This is why I was thinking it might be a CPU issue: writing erratic files on the hard drive, forcing to rewrite the data over and over again (maybe still cerating more erratic files). Could the graphic drivers cause such an issue in any way? Maybe in combination with an anti-virus update?