I’m pretty sure that mega is toast.I lost one to the magic smoke myself yesterday. I think the steppers and a heat bed would thrive on 24v but the arduino board can’t handle it. Did you cut the D2 diode on the Ramps board before you did this? If not then you definitely should to isolate the arduino voltage regulator from the 24v. If you do cut the D2 diode then you can run 24v but you’ll need to power the arduino from a seperate power supply or USB. People do this on3d printer setups with 24v heated beds all the time so lots of info online explaining the process. The 12v power supply is less hassle because you can power the arduino and the Steppers from same supply plus generic laser drivers use 12v too which leaves you set for laser add on later.