it looks like it might be the MOSFET for the main power supply. not sure 100% though.
if the chassis dead, it might be. Check to see if one of the legs of it connects to a little 8 pin microchip (probably an KA3842) if it does that what it is, you will have to remove the MOSFET and get a replacement.
take note though, often with the replacements you will have to isolate the MOSFET metal backing from the metal of the heatsink or you will blow it the first second you power it. The replacement should come with a grey silicone rubber or a clear plastic LOOKING (its actually thin crystalline silica) insulator. if the replacement doesn't come with one, and the original part didn't come with one, then don't worry about it. just a FYI.