Fixing this is a 2 part process. First upload tools.php (it's in the do not upload folder of your package) to your admincp.
Log into tools using your customer number. Under mySQL click "Run Query". Run this query.
Code:
UPDATE setting SET value = 0 WHERE varname = 'enablehooks' and grouptitle = 'plugin'
Now under "Datastore Cache" click on "Options Cache"
Your plugin system should now be switched off, disabling your plugin and allowing you to log into your boards.
You will have to delete tools.php before you will be permitted to log back into the admincp though.
BTW, it would be VERY useful if Jelsoft would provide a 'switch off plug ins' tool in tools.php to make this process simpler.
EDIT2: BTW, if it's only one plugin, why not just comment out the hook line using //