battery disconnected, radio not working when reconnected, help!

on a 99 hyundai accent with a Sony CD player in it. I disconnected the battery to replace the alternator, but when I hooked the battery back up, the head unit is not working at all. No power to it...

this happened once before on this car, but it came back on after a couple days, and had no problems with it since, until now.

Can anyone tell me anything about this?

Maybe you blew a fuse or connected it wrong. Check your power. Try upgrading the 3 different cable sizes that are run alt, battery etc. 2/0 guage or whatever you have on there. Make sure none of the wires are touching so they could short. Just check most everything.