So, Good News: People have been coming to your site and using your mt-search feature a lot! Bad News: They've been using it so much, and you've neglected it for so long that when you go to try and view your "Activity log" the page won't even load.
If you want to reset your activity log, but can't even get the page to load in order to do so, here is what you need to do…
WARNING – This will DELETE ALL entries in your acitivity log! This is essentially what MT does when you click on "Reset Activity Log" – but just wanted to make sure you knew what you were actually doing here.
If you want to review the log first before clearing it out…
Using phpmyadmin – go into your mt database. Click on mt_log table (to view it's details). If you click on the "Browse" tab, you'll be able to scroll through the entires in the log.
To continue on and delete (ie. reset) the log…
Click on the SQL tab. Enter this code in the "Run SQL query/queries on database…etc.etc…" text field:
DELETE FROM mt_log WHERE 1;
Then click the "go" button.
That's all there is to it.