away for a week with he machine off..classic situation which often results from three possible causes
blown motherboard (and poss faulty PSU) due to lightening
loose component (usually memory) due to thermal creep. The week off has given it a chance to settle with a bad connection
bad motherboard due to thermal expansion / contraction - again the week off has allowed a joint to cool and crack
First reseat everything
Next check the output voltage on the PSU
Finally replace the m/b