GS1920-24HP offline

Hello everyone, I'm relatively new to this. I set up my network with a GS1920-24HP. Everything worked quite well for a while. Since noon today, I've been experiencing offline dropouts, although all the port LEDs are still lit normally. The dropouts are quite frequent. I've checked the cables, and the connection to the internet via the router is working. I've also tried restarting and power-off the switch. While keeping it online for short periods, I've updated to the latest firmware. 
What can I do? Thanks for your help.