If the Airport Blue Line Metro is closed you will have three alternatives.
1) The taxi for about 30 euro.
2) The Airport Express Bus every half-hour 24/7/365 to Syntagma Square for 3.2 euro. Monastiraki Station is about 15 minutes' walk from the Syntagma bus stop down Mitropoleos Street.
3) The Suburban Railway system train from the same depot at the airport as the Blue Line, to the Neratziotissa Station, then a transfer to the Green Line train from Neratziotissa to Monastiraki. I'm not sure of the price but it will be somewhere around 7 euro.
If you need to get from the airport to your hotel during peak commute hours, morning or evening, you're better off taking the Suburban Train to avoid the nasty traffic jam.