Is it only your WiFi that it isn't recognizing? Or is it not recognizing/connecting to any/all WiFis?
The simplest thing to try is to 'forget' the WiFi on the tablet, restart the tablet (turn it off and back on) then try connecting again.
Now it is only your WiFi that the tablets are not connecting to, then it may be the ISP (bad signal or internet outage), the modem/router could need either rebooting (just turned off and back on), or it could even need replacing.
On top of this, if the WiFi isn't controlled by you (say it is someone else's, school's, etc.) then the info to connect to said WiFi could have changed. The password could have been changed. It could have been changed to only work at 2.4GHz or only at 5.0GHz. It could have even been altered to block specific devices. In the case of these, you would have to contact the owner of the WiFi.
I have tried all that. Switched off not just a re-start on both tablet and wifi. It recognizes other wifi's just not the one we have anymore and only in the last week. We hold the contract with the wifi provider and the modem is less than a meter away. I thought that the tablet might have had its life being 4-5 years old. Might contact the service provider for a new modem.
I would start with the modem/router reboot and see if that fixes it. If not, then contact the carrier/service provider and have them check your service (make sure it is working with other devices) or replace the modem/router.