1989 Jimmy S15 4.3l just died

I had my Jimmy parked in my garage for about 2 months everything was fine before now when I started the engine everything seemed fine, after about 5 min of running the engine just died. Now nothing happens when I try to turn it over, no click nothing. I can hear the fan go on and the fuel pump seems to click on and there are no trouble codes from the ecu. Just wondering what this could be, when I power the starter manually it cranks. This one has me stumped, any help would be appreciated.
thanks