I have an old toshiba laptop, now it turns off after 20 - 39 seconds?

I have a old toshiba laptop, i k it apart to see if i can fit some new memory and now it turns off after 24 seconds exactly. i have attached all the parts without putting the case on and it is still turning off, any suggestions?
Answer
Check the batteries and power .. problems there can cause boot up failures and unexpected shutdowns...