How bad is it to hook a battery up backwards?

How bad is it to hook a battery up backwards? My jeep died one day and wouldn't start back, went to swap the battery with a spare I had an long story short I hooked it up backwards. The next day my boyfriend tried to hook the original battery back up and it started smoking and sparking. So I'm just trying to figure out exactly how much damage I did. I found out the computer was my original issue, so Im just trying to decide whether it's worth it to try and fix it or if I should just scrap it.