There is a report that up to 6,000,000 LinkedIn passwords have been stolen, and put on the Internet.
The suggestion is that you change your LinkedIn password now.
Read the full story on the BBC link above.
How to change your password:
Log in to you LinkedIn account
Hoover over your name in the top right-hand corner and select "settings" from the menu
You may be asked to log in again
On the next screen click 'Account' near the bottom of the page then go to 'Email and password'
(or I found Password change near the top left-hand side, under my email address)
If you use the same password for other sites you are advised to change those too.