Maybe it went out of scope? Doesn't matter what caused it, it's easy enough to fix.
Open your Bundle Editor
(ctrl-option-cmd-B)
Make sure that the Menu button at the
top left-hand side of the Bundle
Editor displays "Show All"
If it doesn't then click the up/down
arrow and change it so that does
Scroll down until you find the
"Source" bundle
Click the arrow to the left of
"Source" to display the Bundle contents
looking at the Source Bundle items
with green "C"s next to them, find the item
called "Comment Line/Selection"
Click "Comment Line/Selection"
In the upper right-hand corner of the
Bundle Editor, click the "Settings"
button
Next to "Activation", select the menu
item "Key Equivalent"
Whatever is in the text box to the
right, replace it with "cmd-/" (with
your cursor in the text field, press
the keys "cmd" and then "forward
slash"
Verify that the item "Scope Selector"
text area is blank--nothing should be
in there
Exit the Bundle Editor, Restart
TextMate, open a new editor window
(cmd-N) then enter "cmd-/" to verify
that it's working
This command is not limited by scope so it will work in any context/environment/language, but because it is not limited by scope it can be overridden by a Bundle-specific comment command (e.g., a comment command in the HTML Bundle, so, if after you've followed those steps, it's still not working, you just need to find that Bundle-specific comment command and either fix it (using the steps above as a template) or then use the steps above as a template) or deleting it.
While you're at it, you might as well fix the command in your HTML Bundle. From your Bundle Editor, scroll down to the HTML Bundle then click on the Comment item (you said in your Q, you had located it). Click the "Settings" button on the upper right-hand corner, and verify that "Activation" is "Key Equivalent" and "cmd-/" is in the adjacent text box. Next verify that "Scope Selector" is "text.html"