well you need to replace the earth strap anyway; so replace it and see if that solves the problem. Was the strap completely snapped??

it well be the battery has seen better days; putting a multimeter on it while its running will rule out alternator, as anything above 14 volts is OK

what the battery volts after a few days before you attempt to start it?

If its not the earth strap, then i suspect a new battery is needed or you have a battery drain going on somewhere