There could be a number of reasons and without knowing your make and model of the router/modem its hard to tell…but…
Is it broadcasting a wifi signal.?
Look on the modem, there should be lights. One will show if its broadcasting a signal, one will tell you that you are connected to the internet. Then there’s the power light
Consult your manual for these. Your manual should show what’s what
If its broadcasting (or the lights showing) can any device see the wifi SSID. That’s the name of your wifi – normally defaults to the modem make or model
If there’s no SSID showing then it could be that the SSID has been hidden in the modems settings for some reason.
The only way to fix this is with set up the connection to the wifi manually on a device OR connect your computer to the modem via a LAN cable and switch the SSID back to “make visible”. Somewhere in the settings page
If the SSID is visible and you can’t connect, make sure the right password is being used.
Most modems on the back have a switch to turn the wifi off. Mine is a push button.
Has this been pushed by mistake?
However if you’ve reset the modem then I would of thought that this would have been overridden when you rebooted it.
Lastly you could try a factory reset on the modem.
First this will reset the modem and you will have to set it up again to connect to the Vodafone network, so make sure you have all your correct log on details (username/password etc)
On the back of the modem there should be a small round hole that you can press in by using a paperclip or the end of a ball point pen
Again consult your manual. It will show how to factory reset the modem as well as how to set it up again. Just like you did when yo brought it. Remember this will wipe your logon details and you will need to set up again!
Most modems you turn off, press this in for 10 sec’s or longer (depends) and then turn the modem back on and set up as new.