FG disconnected from FMG after uploading new license file

Hello,   My FMG was running with demo license and has an FG added. Everything was working fine. I registered a valid FMG license and after I uploaded the new license file to the FMG, the FG appears to be disconnected from the FMG. It seems the FMG cannot establish a tunnel to the FG. Do you know why this happened and what can I do to bring the FG up again?   Thanks