I didn't have enough money to insure my car so I let it sit for 5 months. I tried it today and it doesn't start. Does anyone know why?
So, first I would charge the existing battery overnight with a mains connected charger. Then I would test the battery to see if is still viable.
Then inspect the plugs and leads (clean and dry), water and oil levels, also if the gas tank is close to empty I would add a couple of gallons of fresh fuel.
Then try to start the car, if it starts then let it idle and warm up without driving it.
Then you should check the brakes, if they are disc brakes they may have some surface rust visible - if this is still light then driving with applications of the brakes will remove it. If heavy then taking the wheels off and cleaning or replacement will be necessary.