Inspiron 1764

Im trying to fix a Inspiron 1764 laptop for my neighbor and have been having strange results. My neighbor told me that she was having issues turning it on. She gave me the laptop and I tried doing everything such as pulling the battery pack, removing the internal watch battery, and even powering it on with out the battery pack hooked on, it just wouldn't turn on. So I left it unplugged and left the battery off for about 24 hours and then tried to turn it on and it worked. Everything was working great and I even was able to update the bios to the latest version. Kept it for a week and would use it on and off (making sure that it was working) with out any problems. I then gave it back to her and and it worked for about a week, and then she told me it wasn't turning on again. So I took it apart and checked all the connectors and cables to make sure that they where plugged in( witch they are). Tried to turn it on and it wouldn't turn on again, left it for 2 days(had work) came back and tried it again and it now turns on. Put every thing back together and it wont turn on. Any ideas of what could be wrong?