This not only DDoS attack, which can turn the program down and disturb its functioning. Internal problems are not rare too, and can also be really dangerous, shaking fragile programs' stability. Here is how administration of Savinvest.com explains why members could not log into their accounts yesterday:
Dear Sav-invest members, there has been a DNS glitch happened today, but it only affected some areas as I speak. some of you may not notice but some may still have problem logging in. If that's the case please try to log in a bit later. So far all the pending withdrawal requests were processed.