Electric Problems

[CMikeB]

I just did a head gasket job on my 1999 Subaru (I know, no surprise there). The very last thing did was to attach the negative battery cable, and when I did I heard a 'pop' coming from the engine compartment. More specifically from somewhere toward the back of the drivers side of the engine compartment.
I started up the engine anyway and it seemed to idle fine for about a minute and a half, then the electronics went dead.
Now I hear the beeping when I first put the key in and my auto locks work, but I have no other electronics. No accessories, no lights etc...
I'm not good with electronics, any suggestions on what I should check first or what could have fried?