You probably just popped a fuse. Make sure you work on the machine unplugged and wait 15 minutes before doing anything and make sure all cables are in perfectly before powering back on.
There are two small ceramic fuses usually labeled F2 and F3. They should be 1.6amp 125v fuses and you should be able to just pop them out and put new ones in. I haven't worked on the SP300I machines very much for some reason so make sure to double check that fuse rating. I'm 90% sure it's the 1.6amp fuse.