does the fan come on high speed?
all the time
problem indicated either the processor or the logic board is defective
you might want to bring to repair shop for replacement
I been working with old G5 mac pro and I have seem that problem many times
just to give you head up, the cost of replacement processor or logic board cost more than $600
is there a reset button that maybe will help?
you could try power on hold down option( alt) + command+ R+ P
keep holding it until the computer shutdown and restart again
give that a try
that will reset the power on your mac
give it a try, let me know if that work for you
if not, then you need to bring to repair center or apple store
hope I have answer your question, please click accept when you done. thank you