1) We assume your PC is turn off! Look for your MB manual and/or locate the jumper right on the MOBO.
2) At this time it is mandatory that you REMOVE the power cord! MSI, my Mobo's manufacturer recommends to avoid clearing the CMOS while the system is on; it will damage the mainboard. Thus why you have to unplug the power cord to be sure no power comes to the Mobo druing the operation.
3) Once located, change the adequate jumper to the position that will reset your BIOS.
4) Wait 20 seconds for it to make its reset job.
5) If you positionned the good jumper on, by now your BIOS should have been completely overwritten and will open with original manufacture settings at next reboot.
6) Change the adequate jumper back to the default position, that will enable normal boot up procedure instead of the one that just reset your BIOS.
7) Stick power cord in, and turn on power switch on power transformator.
8) You are ready to go, by now your BIOS menu should be accessible once again.
If you need them I can provide sample pictures of clear CMOS jumpers location.
Here is a couple
links on the subject