I noticed this yesterday and received a comment from a user as well: MP3Stor appears to be offline.
However, I have also heard other people comment on how they had such a good catalog, and probably won't simply dissappear from the face of the earth, but will instead surface again under a different name (or even back at MP3Stor). But currently I don't think anyone knows what happened to them.
If anyone knows more about this, please post it here.
It looks like payments are disabled for some people at mp3stor for the time being (as well as JustMusicStore) as of 6/24/07. If the situation changes, I'll post it here.
EDIT: I attempted to make a payment from a US IP address, and then a European IP address - and it looks like if you are coming from the USA, you don't have the option of paying with credit cards.
So it looks like - at the moment - if you're coming from a USA IP address, you won't easily be able to make credit card payments on your own. On the other hand, if you have a friend that has an IP outside of the US, you might be able to get them to transfer credits to you. There is an option of "Transfer Funds" that you could use for precisely that.