Did you see this? https://www.dell.com/support/kbdoc/en-us/000124349/understanding-beep-codes-on-a-dell-desktop-pc

6 beeps means a video card or chip failure on that chart.

That's what i wrote. I already looked it up. But i have no idea how to fix it now. Do i have to buy a new chip/video card?

Check all cable connections for a tight fit first--cables can loosen sometimes. Remove them and put them back. A new card does not cost too much for mid range video card. 

If what @Mary G  suggested doesn't help, you may also want to try re-seating the video card in its slot too. And reset BIOS at the same time, which would clear the error, assuming the card hasn't died:

1. Power off, unplug
2. Press/hold power button for ~15 sec
3. Open case and remove motherboard battery (check Service Manual for details)
4. Press/hold power button for ~30 sec
5. Re-seat video card in its slot (#23 in diagram on page 38 in Service Manual)
6. Reinstall battery (Time for fresh CR2032 3-volt coin cell battery? ~$2)
7. Close up and connect mouse, monitor and keyboard
8. Reboot

Thank you for your help!

I will try it. But i have a question? Does my PC even have a graphic card? In that case what do i have to replace?

Sorry if my question is dumb,

Julia

@Julia94  The error message implies you have an (optional) add-in video card. If you have an add-in video card, it will be in the area marked by the green box. If you only have onboard video (standard), those 2 ports are in the area marked by red box.

If you have add-in video card, the 2 onboard ports are disabled, so the monitor(s) has to be connected directly to the add-in video card.

If it turns out the add-in video card died, you could physically remove it from the motherboard slot and connect a monitor directly to either the onboard HDMI or VGA port, until you get a new add-in video card, or if you don't want to buy a new add-in video card.

Using an onboard video port may affect PC's performance. And given a choice, use the onboard HDMI port which is a digital signal, while the onboard VGA port is a low quality analog signal.