Yatse use Kodi Webserver to communicate and control it but also Event Server.
If Yatse can no more communicate with Kodi WebServer then it will indicate that it's offline. (The fact that the arrows continue to work does not mean that Kodi WebServer is working, just that Event Server is working)
There's different cases explaining the offline message :
Kodi Webserver problem:
• Kodi WebServer can crash, so a reboot of Kodi + Wifi router is a good start.
• Sometimes on Kodi update the settings are lost and needs to be adjusted again. You can simply reconfigure Kodi as per the Wiki documentation (Kodi configuration wiki page) and it should work again.
Not connected to Wifi:
• By default Yatse only allows communication to Kodi when connected to Wifi to limit battery usage when not at home. Check you are connected to Wifi, and if you want to use Yatse without Wifi then uncheck the box Wifi only in the host setting.
If Yatse arrows in remote screen continue to work then you are in one of those 2 cases, your network configuration is OK
The other common reason is that your Media Center have changed it's IP address, and Yatse can not find it again.
To solve this problem there's 3 solutions but each one depends on your network or specific computer Kodi runs on.
• DHCP Reservation: This ensure that your Media Center have the same IP address after each reboot. This settings is specific to you Wifi router or internet provider modem. Their documentation should help you with that.
• Static IP: This set up the IP address at the OS level to ensure that it does not change after reboot. This settings is specific to the OS Kodi runs on. A simple google search with your OS and the words static ip will give you help on this.
• DNS: If your network support that, then instead of using a raw IP address in Yatse settings, you can use the dns name of your host, so that Yatse have a way to find all address changes without any trouble.