First check the line. If BT, dial 151.
Check with PlusNet, but not now as they are very busy. Best to raise a ticket.
Common causes of Broadband disconnections:
The USB modem may not be getting sufficient power, or the power is being reduced by another device on the same Hub. View this Microsoft article click here
The computer may be configured to turn off the modem to save power. In Device Manager, find the modem. It may be under Network Adapters. Select Action, Properties, Power Management. Do a similar check on any Routers you are using .
Phone extension cabling may be prone to induced interference from mains wiring. If necessary, run a temporary extension lead to rule this out.
Radio interference from cordless, including DECT, telephones. Turn them all off and disconnect, to check this one.
Radio interference from taxis, CBs, neon lights, streetlights, etc. If one of these is suspected, BT can fit a suppressor to the incoming line.
There may be a clue in Event Viewer. Start > right click My Computer > Manage > Event Viewer > System > Select an Event > Action > Properties. You can send the data to Microsoft for more detailed information.