Looks like a popup saying you don't have internet. Could be from any program needing a connection to the internet. It's almost impossible to know exactly what. Sadly, some programs think it's the greatest thing since sliced bread and the user needs to drop everything to let it update or whatever from the internet. [/cough "Discord"]
It could be a program that needs the internet, but was so poorly written, that it thinks it's permanently off the internet, even if the actual outage only lasted a few seconds. People download all sorts of small programs, doesn't mean the program is a bad thing, but it the program might just be poorly written. There are a lot of micro programs that barely get out of beta before being released to the wild. The program might generally work pretty well, but have some bugs like thinking it permanently lost the internet, even if the internet was only down so briefly, the user didn't even know they lost the connection momentarily.
Did you try killing the popup with the Task Manager?