If there is a TFTP Timeout error shown on the screen this might happen due a stopped service of the TFTP server. 

The error you do get on the client will look like this: 


Open the terminal in the Linux Ubuntu machine and give below command:

$ sudo service tftpd-hpa server restart

If you now command below line it must show in green that the service is active/running*.

$ sudo service tftpd-hpa server status

* older versions of Linux Ubuntu will show a process and 4 digit numer if the service is running.