Starting problem

My Lumina 94 APV has been parked for a couple of months and without knowing ,my battery completly discharged caused by the inside light was left opened.So when I try to start I had to change the battery cause it was not charging anymore and now everytime I start my car I have to crank for about 4 or 5 seconds.And then I have no more problems until the next time I have to restart the car again.Any idea what could be the problem.I also changed my gas filter with no improvement.
Could you please give me some advice.Thankyou