I accidentally locked a note and now I can’t open it.

Unless you know the password, you can'y unlock the note. From the article below.

To unlock a note, you need to enter the password that it's locked with. If you forgot your notes password, Apple can't help you regain access to your locked notes. You can create a new notes password or use your device passcode instead, but this won’t give you access to your old notes. 

Notes - How to lock on your iPhone, iPad, and iPod touch.  *