|emachines computers are well known to have el-cheapo power supplies that tend to fail more often than average, and when the power supply does fail, they are a lot more likely than average to damage something else, often the mboard.|
This is especially the case if it's a BESTEC power supply!!
The technician probably tried another power supply with the mboard and found it still won't boot. There's no need to re-connect a dying or dead power supply.
You can probably buy the same mboard for it used off the web for a reasonable price, and get yourself a power supply. The chances are very good nothing other than the mboard is damaged.
HOWEVER - make sure the ad says the mboard has been tested and found to work!
It's best to get the same mboard that was in an emachines system, that has an emachines bios version, because if you get one with a different brand's bios version, you may NOT be able to do the emachines recovery procedure to restore the original contents of the hard drive for your model with (a) Recovery disk(s) meant for that - the program may quit if the bios version is not emachines.
You can look up which mboard you need here:
You can usually replace the power supply with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.