how do I get my iMessages active on my computer

My iMessages on my desktop computer was working find. For some reason my IMessages now are not working

lately -when ask to sign in I put my apple ID and password in-I get a message that says wrong password. Although I am able to log in online using the same apple ID and password. My iMessages on my phone is working find.

How do I get my iMessages to work on my desktop again.