92 Cherokee stranded at mechanic

Please help me if you can.

I have 92' Jeep cherokee and it recently has hit the wall. I took her in because I had to bet a new crankshaft/bearings that caused a my rear engine seal to leak oil. After spending a a lot of money on that. The cars seems fine. Until

4 days later. Take car to supermarket, come out and its dead. And I mean dead, no lights (inside or out), no automatic doors. Everything is dead. Seemingly, the car did not start.

So I'm thinking the battery is dead. Maybe I left my lights on or something. I come back in a couple hours with jumper cables. Before we hook everything up, I open my doors to see the lights are working again inside (but not in ful).? I hook up the jumper cables and power is restored throughout the car. Wait a few minutes and then try to jump her. Then nothing happens (well I shouldn't say nothing. The battery started sparking a little bit. All the lights go dead agian). But never ever did it start or get near starting

So I take it in. The mechanic says my battery is fully charged. And cannot what is wrong? He mentioned some kinda security system. That could be telling "the internal computer" to not start the car. But I have dealt with this security system before. Usually the lights and everything will work but the car won't turn over. But this is different. As my car simply will do nothing.

Anyone have any ideas? I got my car sitting at the mechanics shop that says he cannot do anything.

All the help is greatly appreciated. Cus im lost .

thanks