jane
What you have to do is depress the bit (that is outlined in the picture below) away from the outside of the knife, towards the recess where the blade was when it was closed. This will release the lock and allow the blade to be folded back into position.
See if this helps:

- The liner lock is outlined in yellow
- GerberOctanelinerlock.jpg (100.38 KiB) Viewed 7272 times
This is called a "liner lock". It is a spring tensioned lock that sits next to the blade when closed, and springs into locking position when opened.
When new, some liner locks can be stiff, but they tend to self adjust after use.