error 500 connection refused

hi all, i have a big trouble
from 10 days a guy bounce me and i really can't run a site for more then 9-10 hours.....after that time if a guy try to connect to the site, the error 500 connection refused appear and i must replace all the system folder with an old one that i have saved....so i can use it again before a new error like this...all it's so incredible..someone can help me please?
(before the bounce, never see this problem)
Thx to all for the help