There could be several reasons for this. If you have a version 3 probe (white modified TP-Link router with a USB stick), your probe may have a problem with filesystem corruption. Please first try this simple procedure:
Unplug the probe from its power source
Remove the USB stick from the probe
Plug in the probe WITHOUT the USB stick
Wait for ten minutes
Insert the USB stick
Note that this must be done on a network with working DHCP; it will NOT work if you have configured your probe to use a static IP address. If you have configured your probe to use a static IP address, the only thing you can do is move it to a network with working DHCP.
If, after an hour, your probe is still listed as disconnected, you may need to replace the USB stick with another one:
Unplug the probe from its power source
Remove the current USB stick from the probe
Plug in a new, brand-name USB stick that is at least 4GB in size (it does not need any preparation but its contents will be erased so make sure it does not store anything crucial)
Plug in the probe
Wait at least 30 minutes for the probe to initialise the drive and upgrade its firmware