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.