Help: my car annot start

Toyota 03, I drived yesterday, it hapen once,but when I try second time keep turn the key on the start position longer time the car start, I didn't pay attention I thought this hapend ocasionaly. but today it is totally can not start, the light can be on, the othoer involved the power is ok. Do you think is the battery problem ? by the way, this battery I changed 4 years age I need the car go to work tomorrow. Thanks for your help.