You only asked what was wrong with the code, and I told you. It is possible that there is no entry in table tbl_users where the value field user_id matches the value in form field Text18, and that could be why no update is taking place.
Bytemyzer is correct about the format being in error (I guess I could not see the Forest through the Trees), but I think that there may be an extra ")" in the code. I do not see an opening "(". Remove the ")" and try again, because a Syntax Error can cause an SQL Statement to fail and not report the failure to the application.
Whoops, you're right. I removed the opening "(", but forgot to remove the closing ")".
swarv should definitely have received an error if he had actually executed the SQL statement from my code, which indicates that he did NOT, in fact, include an Execute statement in his code.