I overclocked my Fx-8120 and now I can't boot properly

Try resetting the bios using the jumper switch. You'll need to figure out were it's at by looking at your manual but normally it's next to the battery.

Unplug the machine from the wall. Then clear the BIOS using the clear CMOS jumper or whatever method your motherboard's manual suggests for clearing the CMOS. If you do that properly your board should be fully reset to default.

If you can get into the bios again.
Check the multiplier of the cpu is set back to the stock value.
Make sure where it states the memory bus it is set back to 200Mhz.

If the memory has an Xmp mode make sure it is enabled in the bios if you did a factory reset of the bios it`s self.
The default factory setting should have XMP memory mode disabled.

Check the core voltage of the cpu. 1.25v to 1.35v is fine for a manuall setting of the cpu core voltage at it`s stock frequency.

To the overclocking bit and why it may fail.

If you had while attempting to overclock the FX 8120 cpu the turbo boost feature still enabled in the bios of the motherboard.

The result will be the overclock often does not work due to a conflict in settings.
It must be disabled, along with the Amd cool and quiet options of the FX cpu settings in the bios.

Most FX based cpu`s will overclock on average about 800Mhz over the stock core frequency.
Above that value, your looking at a core voltage of about 1.40v to 1.45v.

It is important to keep the memory clock at 200Mhz.
Some boards start to add to this. setting the clock to 214 or 216Mhz.

And is the second reason why a board does not boot or post, it`s not that the cpu cannot take the new multiplier setting but the fact that small change pushes the memory speed to far.

Causing the actual none boot of the system on the next restart with the new settings applied.

So you have to keep a close eye on any frequency change over the default 200Mhz setting.
When manually overclocking all settings.

Currently I have a FX 4300, with a MA599X evo R 2.0 on a H60 Corsair water loop.
It helps alot.

With that setup you can get to about 4.9Ghz to 5.0 Ghz with a 1.5v cpu core voltage setting.
At 900Mhz to 1Ghz you start to run into problems, and need all of the power tweaking a bios can provide from a good motherboard.

The option there is set the frequency of the memory lower, then try again with the new cpu multiplier setting, and core voltage change.

It can be dependent on how good the motherboard is also though, the chip set used on the board, and how well it`s power phase is made on the board as to how stable and how high you can overclock a FX based cpu.

A Ga-78LMT-USB3 v4 motherboard is not the best for trying to overclock a FX based cpu to be honest.
The bios, and the options are limited.

So it can cause problems when trying to exceed the 800Mhz overclocking value, most Fx cpu`s with reach without a problem.
A small word of warning do not exceed a cpu core voltage of 1.6v if you have a poor, or a Stock Amd cpu cooler in your system.

You should be using a water loop, or a very good after market cpu air cooler at 1.5v to 1.6v with 1Ghz + overclock of the cpu core.