Either the front cable or the two rears (or all three) are likely seized up.
Hold the release and pull the pedal up, then climb under the truck (Assuming
it's properly chocked of course) with a can of penetrating oil and pull on
the cables to try and find one that won't budge. Worst case scenario is
pulling both drums (They're gonna fight ya) and snipping the parking brake
cables inside the drums. That would get you moving at least.
Another possibility is that the under-dash mechanism has locked up. Unlikely
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