Network connection

Dear All,
Since a few days, the network connection on my machine is off when I start it.
I have to open the network menu, open the used network, close it and get out of the menu. Then, there new initialisation and the network is available.It could be since the last kernel update, but I am not sure.
Any good idea ?
Thanks in advance

Which device do you using? wireless or ethernet?
Please use dmesg and cat /var/log/message to output something for analyze.

I use ethernet.
For the rest, sorry, not used to.
There are effectively plainty of messages in the mentioned file but I am not able to read/open them. Dmesg ?
Thank you so far