What is the exact power supply? Brand + model. When did you install this motherboard? This is a decade-old platform; there are no new motherboards and there's no model I'm familiar with that has 2400 anywhere in the name. Can you double-check what the CPU is? There's no i5-2700, but there was an i7-2700K. But there's also an i5-2400, which makes more sense than a motherboard called 2400.
In other words, let's try to get more information about the specs; I'm not sure about anything you have outside the GPU.
Does the PC work with the old GPU? Does the PC work with the GPU removed and running on integrated graphics?
Welcome newcomer
Hope we can solve your problem!
After you have provided what
DSzymborski
has asked for especially the AGE of the psu with its brand and model. (Make sure to open up your tower and include a photo of your psu or list all information on the name plate its all important)
First off are you sure your cpu isn't the r7 2700 from AMD? It would make sense since the motherboard is new? Probably a long shot but il ask anyway
Im curious to know if your new rtx 3060 ever worked on your setup?
And as mentioned by the previous poster above will your old 1080 work if put back in?
The 1080 should consume more power than the 3060 (+10 watts) and they both should use 2x 8 pin connectors from the gpu so maybe your psu is fine.
Did you make sure to use DDU (Display driver uninstaller) to wipe the 1080 drivers before installing the new card?
View: https://www.youtube.com/watch?v=xn8z39tiEL0
^ this is a tutorial above. I havent watched this video but it was the 1st hit on youtube so im sure it will be easy to follow!
Hope this helps