Well if you have read their email, you know this is prety serious.
ALL activity has been halted. There even is a link to get your money back (which wasn't working).
So yes, it's time to worry now.
The time to worry is before any loss, or least it pays better then. I dont really blame any party as I think we know in concept an exchange with all money centralised is vulnerable increasingly with its success and growth. Bitcoin itself has to avoid this kind of natural crush towards amassing all power and risk in one place or it'll fall into the same problems.
We could say oh the people who ran the exchange are the bad people they made mistakes but really I think the problem was allowing worth to be placed in one place that goes against why bitcoin succeeds in the first place.
Many are suggesting other options other than the 3 mentioned above:
4. Spread the loss to all users in the system and allow trades to continue.
From the blog itself I think it would be best to press on, take the hit and place IOU into accounts that have fallen short from this failure. Nobody will be happy or grateful but it will be a working system at least, its really no good to freeze an exchange and everybody loses. Its like a golden rule really that all exchanges small and large avoid trading halts for anymore then brief periods.
I cant remember which exchange did this previously after a loss, were they able to recover maybe someone can say