Back up your data files and Run the Test and Correct Utility. See the post: Test and Correct Utility.

If you still can't access the account, you may need to rebuild your account index. See the post: Some of my accounts are missing!. Although the problem listed is different that yours, follow the instructions in the post to rebuild the account index or the instructions in the remainder of that post if your problem isn't resolved by rebuilding the account index.

Please reply to this post if you are still unable to access the account.