I viewed a cached (from July 8) copy of your site from google and can see that it is running mojoportal 2.3.6.5 which was released in April 2011. So apparently your site has been running a long time before any problem happened which does not suggest a problem with the code, but it sounds like your site possibly got hacked in some way or another recently.
The link you posted was from a much older issue in mojoPortal 2.3.4.3 which was fixed in version 2.3.5.2 in the fall of 2010.
A DDOS attack is when lots of computers make web requests to overwhelm a server, so it is not caused by mojoPortal code. If someone believes there is a performance problem in mojoPortal that makes it easy to overload a server with only a few web requests then details of how and why they believe this would need to be provided. But there is certainly no code I could write that makes other machine attack a server with ddos.
What I would do is make sure you change your ftp password, upgrade to the latest mojoPortal which may help because if any mojoportal files or js files have been modified to hack your site then replacing them should resolve the current problem.
So I would download your site for a backup and for further analysis of what happened then I would upload the latest mojoPortal files, and restore any customizations to web.config
Then follow the steps in post installation checklist and read up on securing mojoportal.
I would also make sure that no other server side technology is enabled in your site except asp.net , ie make sure php is disabled if it is also installed in your hosting.