It is never a good idea to let any electronic device set unused for a long period of time. I suggest checking to make certain that all of the components are seated properly. Remove the video card and clean the contacts on the card and in the slot. If the computer has any moisture inside these can be corroded. Also, check the contacts on the memory. Be extremely careful when cleaning the contacts. Any ESD will damage them.
I am not an HP Employee!! Intelligence is God given. Wisdom is the sum of our mistakes!!