if the battery has been discharged all the way then quite often this can damage the battery so it will no longer hold a charge
So I'd suggest to try replacing the battery with a new one and also check the state of the engine earth connections as they should be bright and clean
if you can get the car started like this then also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.
if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement