Used to work in a company that used nothing but Dell (older machines where HP) Anyway do you see any message on screen, hear any beeps or reseating the RAM again in its slot(s)?
Just because ram is working in another computer doesn't mean to say ram is sutable for your machine unless it was the same model.
Failing that swap the 2 sticks around i have a acer that does this if i remove the ram and place the sticks back in another slot it wont power up. Now if i place the sticks back in the slot they come from it powers up oh and thats a acer 6920 about 6 months old.
Can-t-get-Dell-Inspirion-E1505-to-Start Put the Ram back where it was and clear cmos. Changing RAM often corrupts cmos especially if new ram is not computable.