1) Open up your terminal (ctrl+alt+t)

2) Run the following command in the terminal:

gsettings reset-recursively org.cinnamon (THIS IS FOR CINNAMON)
gsettings reset-recursively org.mate.panel (THIS IS FOR MATE)

3) Hit Enter

4) Taraa!!! you should have your panel back to their default again.

NOTE :

• you may login out and login in again to get them work again, in some cases you need to reboot your system.

• tested and worked on my MATE Dekstop Environment.