Sounds like the lever spring failed, which isn't uncommon on an old Hepburn, or a bad new spring. It's a double bend spring that sits under the barrel and can be easily see when you remove the forearm. They can be a little difficult to replace the first time you do it, and requires a tiny screwdriver tip to remove the screw in the lever to remove the lever. Then just remove the spring retaining screw and install the new spring.
DZ Arms sells the new springs about halfway down this page:

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TN Longhunter wrote on Apr 8^th, 2025 at 12:46pm:


With multiple Hepburns it wont hurt to have a spare. I have a spare also, but that just means I'll never break the spring. So far in decades of shooting mine I've never broken a spring. I did replace one because someone made a bad spring out of some flat steel that was too wimpy.

ther is a screw in side of action that needs to be removed i also had this problem made my own spring quite a job