The Registry key has probably become corrupt.
Go to Start | Run and type
regedit
and navigate to this key:

HKEY_CURRENT_USER\Identities\{GUID}\Software\Microsoft\Outlook
Express\5.0\Block Senders

Right click it and select Delete.  (Make sure that you only selected Block
Senders.)

Then you will have to rebuild your list.

Any chance you have more than 4,096 entries in Blocked Senders and Message
Rules combined? That is the limit.

IMO, Blocked Senders is a waste of time since spammers constantly change
their addresses. While message rules has its limitations, they are more
effective, especially with an Address Book Only rule, (if feasible for you).

Start with the Address Book only rule and as you get messages you want from
people not in your Address Book, (E-mail from businesses, bills, your ISP,
etc.), you can add them to the rule.

Tools | Message Rules | Mail | New.

Box 1 - Where the From line contains people {check it}

Box 2 - Check: Delete it & Stop processing more rules if you want messages
not in the Address Book to go to Deleted Items, or you can create a Junk
folder and direct them there.

You can choose Delete it from server, but be careful as you will never have
any messages downloaded to any folder if you choose this option.

Box 3 - Click on the blue words Contains People

Click on the first address to highlight it. Scroll down to the end and hold
down the Ctrl key while you click on the last address. This will highlight
them all. Right click on them and click on From in the menu that appears.

Now click Options and Choose: Message *does not contain* the people below in
the top Options box, and choose: Message matches any one of the people below
in the bottom Options box.

OK your way out of the new rule.

Now, only addresses you entered will be downloaded to your Inbox. You will
not see any other e-mail.

Don't forget, if you add someone to your Address Book, you will have to add
them to the rule. It will not be added automatically.

Be sure to see these links for more options.

Some Message Rule Ideas:
http://www.mindspring.com/~majik/messagerules.htm

Some tips:
http://insideoe.tomsterdam.com/tips/rules.htm

Controlling Junk Mail in Outlook Express:
http://www.microsoft.com/windows/IE/community/columns/junkmail.mspx

Message Rules not working?:
http://www.tomsterdam.com/insideoe/faqs/why.htm#rules

You really expect the Block Sender list to do any good against spammers
that change their bogus e-mail address every time they spew their crap?