Have the electrical system checked for a drain on the battery. There will be a little bit due to the computers' memory circuits, but the industry standard is 35 milliamps, (.035 amps). At that rate the engine should start after sitting for three weeks.
Look for a glove box light not turning off or aftermarket audio equipment wired incorrectly.
Also check the charging system to be sure it is recharging the battery. If it is not, you can charge the battery with a battery charger, then the engine should start many days later if there is no drain on it. Once the engine is running, use an inexpensive digital voltmeter to measure battery voltage. It must be between 13.75 and 14.75 volts. If it is low, you have a charging system problem.