If you can "see" the BTWifi signal and your device can select the signal but you are unable to log on to the signal then do as has been suggested above.
If you are unable to "see" the BTWifi signal and as such you can not connect to it the following applies.
BTWifi hotspots are "supplied" by BT Broadband customers transmitting the signal from their home's BT hub or as a commercial hotspot.
The vast majority are from BT Broadband customers. If the home owner decides to switch off the BTWifi hotspot or if there is a problem stopping the BTWifi hotspot from functioning there is nothing that BT can do about it.
As regards the BTWifi hotspot that you were using, if it is a commercial BTWifi hotspot you could point out to the company that is broadcasting the signal that it is not working and they might reset their BT hub which will possibly restart the signal.
If it is a BT Broadband customer, unless you know the home owner who is broadcasting the signal and wish to ask them to restart their BT Hub, which they are under no obligation to do, there is nothing you can do about it other than find another hotspot.
