Hi and welcome.
Firstly make sure all the power cables are attached to the motherboard, that's the 24 pin and the 4 pin 12v ATX near the CPU.
I would suggest first clearing the CMOS and see if that helps. Remove the power cable from the mains supply and then press the power switch on the case for a few seconds just to drain any residual energy in the PSU capacitors.
Once done remove the motherboard battery for at least one hour before replacing it.
Next plug back into the mains supply and boot.
You will now need to enter the BIOS by pressing DEL and load Optimised BIOS Defaults.
Make any other changes to the BIOS settings to suit your self like disabling the floppy drive, disabling the full screen logo and making the HDD the primary boot device and then press F10 to save and exit.