First you must use your impulse login. If its not working, its because you bought demigod from a different email adresse than your email you use for impulse. There is a fix on the forum for this that allow you to merge those email.

I dont remember  the link so search the forum a bit.

This will begin a waiting period. It can take some time to come back to life after you change your user name.

Make sure you are using the correct login.  It's not an uncommon mistake to use the wrong login because of the way that Impulse is set up.  (See post above).  If you were able to login to multiplayer in the past and are now unable to do so, there is a file that you can delete in one of the folders to solve the problem.  This happened to me and after waiting a couple hours for it to resolve itself, I did some digging on the tech forums to find the solution.  I don't remember exactly what file it is, but dig on the tech forum and you will find it.