inspiron 6000 not starting up

i bought a dell inspiron 6000 laptop from the pawn shop. i figured with it bein only $600 itd be worth it, of course it didnt have the warranty. i messed around on it last night just fine. i left it plugged into the charger all night cause i ran the battery pretty low. i went to hop on it this morning and it wont turn on at all. if i hit the button, the green power light will come on for about 4-5 seconds and go back off. ive even held it down for a few seconds and itll come on for the 4-5 seconds, go off and come right back on. during that, the laptop is silent, no fans or anything running. ive taken out the battery pack and just plugged the laptop into the wall and it still does the same thing. i put the battery back in, the little battery light on the laptop came on for a second then went back off. since then ive not been able to get it to light back up. im guessing the charger cord works fine since the green light on the box stays lit.

my question is, is it the battery? or something more complicated. and is it cheaper to fix this one up or buy something else brand new.