Assuming no transactions have recently gone wrong (eg. resulting in too many unpaid item strikes) it could be that there has been suspicious activity on your account. How careful are you about phishing e-mails?
It could also be a genuine mistake. I recently got an e-mail from Ebay suggesting that a seller I had bought from may have been dodgy and that my bid on his item had been cancelled (I already had the item!) However, a couple of days later I got an e-mail from saying that it had been a misunderstanding.
All I can suggest is that you keep e-mailing Ebay. They should really tell you why they've suspended your account.
